And how often does a character who can use great weapons get Advantage?
There's a reason why we have a party. And a party that we fully control, so unlike PnP we can make sure our casters don't just use Fireball every turn. And getting advantage on attack rolls is something that you should already prioritize, even if you don't have GWM in party.
Ways of getting advantage:
1) attacking enemy while hidden or otherwise unseen.- We still can hide with a bonus action as any class (BA hide should be removed imho);
- Attacking target without darkvision from shadows, while having darkvision ourselves;
- Invisibility grants advantage on attack rolls (however we need Greater Invisibilty to make it reliable).
2) attacking prone enemy when within 5 feet.Some ways of knocking enemies prone:
- frozen surfaces,
- Grease,
- Wildheart Barbarian Eagle and Stag special attacks,
- Beastmaster's Boar companion special attack (it's spammable, long line, with decent DC),
- Battlemaster's Trip Maneuver,
- Topple attack from Quarterstaff (14 DC, not attack roll, try it out),
- Tasha's Hideous Laughter,
- special weapon abilities that knock prone (Warhammers and Mauls have Backbreaker special ability and Intransigent Warhammer has Impulse Blast).
3) attacking blinded enemy.- Fog Cloud, Darkness spells and Darkness arrows create clouds that blind enemies inside. You can place them in a way that a melee character could blinded enemies with little trouble. You can also build a Githyanki Warlock to actually stay inside darkness and attack with your Greatsword, could be fun;
- Blindness spell, Raven familiar and Beastmaster's Dire Raven, Absolute Night spear can blind enemies.
4) attacking restrained (emwebbed, ensnared, entangled) enemySources of restrained:
- Web spell, Beastmaster's Spider. Give your melee character Spider Step boots and profit;
- Entangle spell from Druid, and Ensnaring Strike (Ranger, Sorrow glaive, Nature's Snare quarterstaff).
5) attacking stunned, paralyzed or petrified enemy.- Hold Person is the only way to get one of those conditions, so not much here. We need to wait for higher level spells (and monks).
6) other sources.- Off Balance condition from Rush Attack (Halberd, Longsword) or Flourish (Shortsword, Rapier) special weapon abilites.
- first attack against enemy hit by Guiding Bolt;
- Invoke Duplicity from Trickster Cleric (but who would have trickster cleric in their party?);
- Reckless Attack (just one attack per round in BG3);
- Attacks rolls against targets affected by Faerie Fire.
TLDR. With a little bit of thinking you should be able to get advantage for your melee combatant for a lot of attacks. 100%? Not really. 50%? Definitely. And you still can just cast Bless every combat and increase your hit rate that way, or throw acid around.
