Yes it performs a full install of the OS up to the latest patch -essentially factory settings so you would need to relog in ect
GTA Online should be saved to servers (i think the online portion is atleast - not 100% sure but usually online progress is saved via the server not the savefile but dont quote me on that for GTA but for example Overwatch is stored server side for progress)
Everything else will depend on save wise if you ever used rest mode with psplus it will auto upload saves to the cloud, but games ect will need reinstalling/downloading.
However if as above it would look to be the only option for you.