I'm not sure what you mean by 'gift cards won'. I personally don't think that presents count (like, I'm not going to try to put a value on all my christmas presents and pay the church 10% of that). And really I think tything is up to each member as to what they think is fair. Pray about it, and when you get on the same page as the spirit about what you should be paying on (or counting as your increase) then do that. Others may disagree with me. That's just what I do.
I guess I should say, I take 10% of what is deposted into my checking account every month and pay that for tything. I won an Ipod in a drawing once and it didn't even dawn on me to wonder if I should tythe on it or not.
"Counsel with the Lord in all thy doings, and he will direct thee for good; yea, when thou liest down at night lie down unto the Lord, that he may watch over you in your sleep; and when thou risest in the morning let thy heart be full of thanks unto God; and if ye do these things, ye shall be lifted up at the last day."