By the second half of the 19th century, specifically in 1864, the parish priest of Gatão, Domingos Alves da Silva, in response to the inquiry conducted by the diocese of Porto, refers the Church's good state of repair. The good news continued in the following century: in 1937, a series of frescoes was discovered by Father Manuel Couto and, later, Aarão de Lacerda published an article about them on Prisma magazine.
It was precisely the discovery of these frescoes that triggered the intention to classify this Church as a National Monument. In an official letter written by the architect Baltazar de Castro to the engineer Henrique Gomes da Silva, the Director-General for DGEMN – Direção Geral dos Edifícios e Monumentos Nacionais [General Directorate for Buildings and National Monuments], he pronounces himself precisely in favour of the classification of the building and its frescoes, considered as being "precious".
In the following year, the classification proposal, "including the frescoes paintings that decorate the walls", had already obtained a favourable report from the Junta Nacional de Educação [National Board for Education]. Despite "its naïve simplicity, and the fact that its construction is so disrupted, it was considered that the worth of Gatão is a result of a few "affresco" paintings that decorated the Church during the 16th century and that it still keeps, despite the misfortunes it went through, thanks to the high and loving interest of the parish abbot".
So, it was by Decree no. 30762, published in the Government Gazette no. 225, of September 26th, 1940, that the Church of Gatão, together with its frescoes, was classified as a National Monument. Therefore, the necessary conditions for its preservation were created and included, precisely, the implementation of a major restoration intervention, although there were different views on the situation.
The memorandum regarding the intervention of "Reconstruction, Cleaning, Restoration and Treatment of the Frescoes of the Church of Gatão – Amarante" dates back to July 26th, 1941. Based on the building's poor state of repair, the required works were then itemized; these included the reconstruction of the external sacristy, which had a slightly offset position when compared to the existing one (although there was a decision to lower its walls instead, "to free the Lombard band found on the chancel's cornice"); improving the access to the high choir with a new staircase on the inside and, on the outside, using the service staircase that already existed "and was embedded on the front end of the nave's lateral wall"; this wall was supposed to be repaired (but there was a decision to demolish a staircase "that was already old and gave access to the choir, through the galilee's (south) lateral wall").
Besides a general improvement of the Church's liturgical equipment, which included the pulpit (that was provided with a new wooden parapet) or the design of a new wooden cabinet for the choir, it was simultaneously decided to remove the "simple wooden altars that sided the triumphal arch" (...) "to the outside of the church, due to the lack of recommendable conditions"; the same happened to the chancel's altar, "also made of wood and belonging to the same type, to free the primitive granite altar whose table remains intact".
As we can deduce, this was quite a purist restoration. As explained by D. João de Castro - the author of the text found in the Bulletin of the DGEMN – Direção Geral dos Edifícios e Monumentos Nacionais [General Directorate for Buildings and National Monuments] dedicated to this intervention -, "a constructive unity that was necessary for the [...] conservation and aesthetic dignity of this building, which had irretrievably lost its architectural unity long ago", was retrieved after the restoration.
Here we should note that, during the execution of the works, the same parish priest, Manuel da Silva Couto, addressed the responsible services with a rather original proposal for its time: taking advantage of the building work atmosphere that the property was experiencing then, the parish priest requests an expansion of the Church, asking if "the main portal could be moved forward and form a façade according to the church's style. The church's body would become too long in proportion to its width and to the chancel. But couldn't there be, for example, an inscription justifying this anomaly for worshipping needs?" The parish priest claimed the need to have space for the devotees and that priority should be given "first of all, to worship, which is the purpose of the church". This request was not granted.
One of the most extensive interventions that it was necessary to carry out was the full replacement of the Church's roofing system. The memorandum of 1941 mentions the complete reconstruction of the roofs, including new wooden planking as part of the intervention. However, during this work stage, it was felt that there was the need to demolish and rebuild the Church "halfway up the nave's (south) lateral wall", taking that opportunity to replace "the big window, which had been opened in modern times, with two properly located crevices".
Besides the consequent change in terms of the elevation's legibility, this intervention becomes even more significant due to the fact that it gave rise to a protest by the parish priest of Gatão. On December 14th, 1942, Father Manuel da Silva Couto mentions the fact that the work had been suspended. Considering that only the foundations for the new wall had been made, the Church was "going through the winter in this state", without roof tiles and with a demolished chapel, since October.
According an explanation given by the architect Baltazar de Castro, that interruption was due to the fact that the funds required for the roof repair works had been channelled to the demolition of the nave's south wall, which was threatening to collapse. Besides, the purchase of wood for the roof framing drained the initial allocation. The granting of a new allocation enabled the works to be quickly resumed. The Bulletin that recorded these works was published in 1951.
In 1966, there was the installation of an electric wiring system in the Church, at the initiative of the new parish priest of Gatão, José Augusto de Sousa Marques, mentioning its "advantage for the devotees, during the acts of worship" and the fact that it would highlight, "in the eyes of tourists who visit the church, certain parts of its interior, which weren't even able to be illuminated by sunlight". The comment made by the parish priest regarding the dimness that had filled the interior of the Church of Gatão until then, lending it a "heavier, nobler atmosphere, more consonant with its centuries-old age" is, however, interesting.
In the second half of the 1970s, several conservation works were carried out, including the construction of new doors, the cleaning of the roofs and the reconstruction of mortars on the main façade. Several works were also carried out in the monument's protection zone, namely in the parish residence and in the cemetery, which was expanded. In the 1990s the Church of Gatão was already in need of a new plaster coating.
With the integration of the Church of Gatão in the Route of the Romanesque, a general conservation of the Church was carried out in 2015: at the level of the outer vestments, roofing, access doors and lighting and ventilation openings, and mural painting.