My main character is BG1 was a mage with a dexterity of 3. I used the gauntlets of dexterity to raise this to 18. Is this item, or an item that does the same thing, available in BG2, or will it come along with my imported character?

Well when you import your character it will appear naked so no, you wont have the gauntlets on it.
And yes, they appear in the game but i dont think it was too soon.
MINOR SPOILER
>
>
>
>
> If you want to lower an attribute to max out the others, i would recommend strenght because there are many objects that raise it and some of them appear soon.