Which program do you use for pixels?

[Vipershark]

Yes, assuming you're not opening a previously saved gif.
I'm on my phone right now so i cant check but if you open the frame properties there will be a little rectangle (either white or black, i dont remember the default color) and clicking it will allow you to change the color ro whatever you want. Then just make everything you want tranparent said color.

If you're trying to edit a previously-saved gif, you're locked to its color palette and transparency color (usually black) so you'll have to copy and past each individual frame into a new file to fix it