Can`t give an explanation because I`m not familiar with how this GMT works, but there's got to be an answer somewhere! Does it have 2 cannon pinions? I`d need to get my head around how it works to fathom out why it isn`t working..........taking it apart might be the best option.
I`m sure you've thought of this but I`ll mention it anyway; is the GMT hand a tight enough fit? It's usually obvious when refitting hands whether they're loose or not so it seems unlikely.
We'll await the answer explanation in due course
Paul