@enfors The PF2E remaster naming scheme can sure be a bit confusing! The books are both good and contain almost the same content, but the GM Core provides the most recent rules update (however it does not include all of the variant rules presented in the Gamemastery Guide).
For further reference, the old books:
- Core Rulebook
- Gamemastery Guide
- Bestiary
- Advanced Player's Guide
The equivalent remastered new books:
- Player Core
- GM Core
- Monster Core
- Player Core 2