You can open the images in Photoshop and take out the white background there by clicking double clicking on the layer -> blending modes, and play around with the sliders at the bottom, it's a super fast way to do this. but you may have to do it with every frame.. A gif is a sequence of images, and if those images are imported at JPGs then they definitely won't have a transparent background. PNGs do transparency, JPGs don't. Not sure if that's helpful in your case but something good to know.
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