We currently offer an almost indefinite selection of customizations for avatars at your disposal. Unfortunately, Gather does not support self-made or customized avatars at this time due to how avatars are stored and retrieved. This is a feature we would like to implement in the future; however, it is currently on our list of longer term goals.
You can upvote this request or add a comment with details about what you're looking for on the Custom Avatars thread at https://feedback.gather.town.