I don't mind GIFs in avatars because that's out of the way from the body of text the user is posting and as such is not distracting.
But with Sig's they're designed to be the last part of the text that a user posts and as such will be highly visible.
I just find GIFs annoying when in Sigs and to an extent they make the forums look worse by convoluting the discussion with distracting animations.
Think of a Sig as something personal to you as a user:
- An image that you have created (we all know there are some great artists on these forums!).
- A favourite/funny/thought-provoking quote.
- If advertising, then a link with the Logo of the Site/Shop/Service you're offering.