To be on the safe side, never say you never use any of the products they ask about, since that will probably make you the opposite of what they’re looking for.
If you don’t use the item in question, it doesn’t serve anyone by lying about it. Yes, this makes you less likely to be selected, but that’s the way the proverbial cookie crumbles. If they suss out your non-use during the actual group, that’s going to put you on the $hit list.
… it’s hard to remember how much you spent [from a gift card], since those cards don’t offer a way of checking your balance.
I know this isn’t the case for all of them, but the last focus-group payment I got was on a Bank of America gift card, and it offered the option of free cash withdrawals. I simply went to the nearest BofA ATM and cleaned out the card in one swoop. Even if the cash pulls were a small fee, I think I’d still do it to avoid the hassle of keeping track of the balance.