Do you still want/need the GF-XXX ticket tag? If not, you can try a rule event of Ticket Merged to remove the Ticket Tag(s)-make it blank. Or change it to a shorter one.
Removing from the table caption after a ticket merge but still have it show before a ticket merge might be hard to do. Maybe a script that would count characters and return blank when a reached a certain number? I’m not sure on that one.
Try this: [=F(TN('{REPORT TICKET DETAILS:T.RemainingAmount.sum:(TEN.Table=$1) and T.RemainingAmount>0}'),'0')]