Interesting question and your presumed answer for discussion.
I'll start off with the premise that the engine and gearbox are rated for the design torque and temperature.
If you exceed the turbine inlet temperature (too much fuel or too little air) than the turbine blades will rapidly detriorate. A small accunulated amount over hours or within a minute or less.
Torque needs two parts, driving torque and load. I presume that the driving torque from the turbine (typically a free turbine) is already sized to the gearbox in normal operation. If for some reason you managed to greatly increase the load torque (typically pitch changes on props) beyond design limits the turbine would slow down way earlier than a gearbox failure. This may have the result of a back pressure on the engine compressors and a droop in operating RPM which in turn results in less driving torque and may result in the engine becomming unstable.
I suppose that barrit1 when may some better words on this