There is no way to legitimately acquire the Girdle of Masculinity/Femininty in Baldur's Gate 2. It isn't available in game, and most items cannot be imported as the start of Shadows of Amn takes your inventory.
It is still available in the game files, so either spawning it with console commands (BELT05) or a save game editor such as EEKeeper can add it to your game.
According to multiple posts on the Beamdog forums, Dagomir has resistance to non-magical weapons. You've essentially confirmed this, by hurting him with a spell but no melee attacks.
You'll need at least +2 weapons, to be able to even injure him. According to a thread on Steam, you should be able to buy some at the Copper Coronet. You may also be able to "create" magic weapons using the spell Enchanted Weapon.
Best Answer
https://forums.beamdog.com/discussion/13885/durations-descriptions-list-of-issues
1 day in game = 24 hours in-game = 1200 rounds = 7200 seconds
1 hour = 5 Turns = 50 rounds = 300 seconds
1 turn = 10 rounds = 60 seconds
1 round = 6 seconds
I presume cast time is in rounds and that duration turn is upon the person's turn.
Notice: Some spell time/duration description are inconsistency in defining time/duration so beware.