I wonder which varieties of 60 DTM tomatoes you're growing? Many toms with a nominal dtm of 60 days are also selected to be tolerant or well adapted to early season temperatures, so they may do better simply planted out earlier as KD suggests.
I think the reason for having a DTM "from date of transplant" is to acknowledge the time it takes for tomatoes to develop enough root system to pump out the fruit as well. You are providing some extra root space with a 1 gal pot but it's not enough for full production to kick into gear.
Larger pots would be another option, but then you would need a bigger greenhouse.
