I am sure it is not dramatic as this is a simple movement, it may be a broken pivot or shock jewel, where are you in the world?, is this a swiss or asian movement and has it ever been serviced
Well if you want to sweeten the deal, I offer to look at the 026 for you and get it working if possible for free
and i can have look at the broken brietling and make you an offer for it depending on the damage to that. I am in the EU
Just dont get too drunk the night before, on mine if it was not that we had a police escort for the column of bikes we would have all been pulled and banned from driving!
Put the gmt wheel on the movement with out the dial and try it that way and see what happens that will rule out the movement if it is fine then and will indicate something out side the movement causing the problem
it cauld also be the slipping clutch wheel for the GMT?
do you have a friction washer between the GMT wheel and the dial? it may be that the gmt wheel gears are not meshing well if the gmt wheel is not held down or the dial may not be central casusing the gMT wheel to bind on the dial center hole and causing the gears to slip. At this point i would be checking this as a gear jumping will cause bits to wear off and cause a stopage or damage to the movement.