Google Earth Pro isn’t really designed to be used offline. You can set the local cache to be quite large and if you clear the cache, then explore the region you want to keep offline, the the program will store a small amount of the maps in the cache. However, elevation, terrain, and location data is streamed live, so GEP isn’t much use if you need that data offline.
You’re probably better off using a dedicated mapping application used by the aviation industry, but that will cost (naturally).