GTA 5 disc on PS4

I know you need to have PS+ if you're buying game from PSN to play it

No, this is wrong. You can purchase games on PSN and play them without PS Plus.

This handy chart (from 2015) found on Game Fanatics illustrates what PS Plus offers:

Not much has changed since 2015. The most important changes being the release of the PS5, and the discontinuation of Instant Game Collection for PS3 and PS Vita in 2018.

As you can see, you will need PS Plus to play online multiplayer games on PS4. It doesn't matter if you bought the game on PSN or as a disc.

There exist a few exceptions to that rule, though, according to the official Playstation site:

Do I need PlayStation Plus to play Fortnite?

No – Fortnite and other free-to-play games such as Apex Legends, World of Tanks and Spellbreak are free to download and play online.

Since GTA 5 is not a free-to-play game, PS Plus is necessary in order to play online.