If it uninstalled, it probably only removed itself from the library, all your files may still be intact
Also, i don't think there's a way to update Gmod to Steampipe as it should have already been updated to Steampipe, unless you mean getting it in the common folder instead of the username folder, in which case, the game would have to be uninstalled anyway (but manually uninstalling may delete the garrysmod directory in the username folder, so it's best to backup your files)
