Due to differences in the versions, it's not possible to play GTA Online on PS5 with players on PS4, unless you're still running the PS4 version on your PS5.

Secondly, even though you were playing GTA 5 on PS5, you’re still just playing the PS4 version. That means if you keep playing the PS4 version, you’ll still be able to play with PS4 players.

How to move your existing GTA V save data from Xbox One and PlayStation 4 to the new updated versions on Xbox Series X|S and PS5.

How to migrate your GTA Online character from PS4 to PS5, and to Xbox Series X, so you can continue your criminal enterprises in the enhanced version.