All players receive the regular Battle Pass for free each Season. You'll earn rewards on the regular Battle Pass as you earn XP from missions.
The Golden Battle Pass (GBP) can be unlocked with GP and will provide additional rewards along with rewards from the free Battle Pass.
You can unlock the Golden Battle Pass from the "UNLOCK ALL GOLD ITEMS" menu. Purchasing the GBP will also provide an XP boost and add rewards from the GBP for levels you've already completed.
There is also the option to auto-complete both the regular and Golden Battle Pass listed as "BUY PASS + ALL LEVELS". This will complete both passes immediately and add any rewards you have not yet collected from the passes to your account.
And keep an eye on the timer! The timer under the Battle Pass counts down to the end of the current Season.