After seeing "the gimp" character in Pulp Fiction, two questions come to mind.
- How does "the gimp" come to be in Maynard & Zed's basement? and
- Why is "the gimp" covered up with a mask?
'The Gimp' was already in the basement, he is kept down there. We can infer this from Maynard stating 'The Gimp's sleeping' and Zed suggesting 'Well we better wake him up then'.
To answer your second one, I suggest googling what a 'Gimp' is. However, if you don't want to, then I suggest googling 'gimp mask' which should be able to explain it.
This is all about sexual practice called 'S&M' which stands for 'Sadomasochism'.
Hope this helps, apologies if my quotes aren't 100% as I don't have the script to hand.
He could easily have been kidnapped just like the two main characters were. If Butch had not gone back then Wallace probably would have been used as a gimp too.
However as Butch's father's friend told him when he was a child "When two men are in a dangerous situation they take on a certain responsibility for each other" (quote might not be accurate) he was honour bound to go back and prevent this.