Tops sinking is something you tend to see on early 50's to about 56-ish Gibsons, the tops were very thin between those years, and I've seen quite a few that were losing their arch. This one looks like 60's (right?), tops tend to be thicker on those and don't usually develop problems.
You'd want to check if the neck is still at the right angle, still firmly attachted to the body. That's a problem with quite a few thinline hollowbodies - short heel, less secure neck.