I have money left on my gift card from Christmas last year. I wanted to use it to buy stardock games such as Rebellion, but I'm guessing now that they sold off Impulse, my money went with it ...

What most likely happened is due to the terms of your gift card you must have paid holding fee's for the money in the form of credit on your gift card *assuming its a visa gift card* changing your balance.

If its such a gift card check the site on the back of the card it will direct you to a place where you can view the remaining funds and possibly set up a account etc to help with future gift cards from the same company/issuer.

Also I am not familiar with stardock gift cards or ones specific to impulse. You will have to read the terms on the card.

~~good luck!