Monk and Great Weapon Master interaction question

Apologies right away as I'm a new player. I watched a Dungeon Dudes video about Monks and they explained that a Monk could take a feat in Great Weapon Master or one level in Fighter to get the Great Weapon Master fighting style and that will allow your Monk to Dual Wield. What I could understand was that this would essentially give you an extra attack with your off hand (or maybe just make it more effective?) would someone be able to clarify if that is how this works in BG3 and if it's even worth considering?

For what it's worth my plan would be to play to level 20, 16 levels of Shadow Monk, 3 levels of Assassin Rogue and 1 level of Fighter. I'd do the Tavern Brawler/Strength Potions build but am unsure what to do with other feats if anyone has a suggestion. Thanks!