You'll get a supercar with an early-access upgrade, three clothing items, waived LS Car Meet membership costs, and increased incentives for two race series in the first month, among other things. Benefits must be claimed before they expire.
Rockstar stated that events will continue "as usual" for all players, so you won't be locked out of critical content if you want to play for free. But it's unclear whether GTA+ is a good thing. While it may be a better value than paying real money every time you want an in-game cash increase, it may also put you at a disadvantage if you can't afford the monthly cost or a hardware upgrade.