I agree, often I have found my self wanting some way to either set gimballed to a fixed variant or easily clear targets, right now trying to drop a target is a pain, and you may just end up selecting a new target by mistake.
I have thought this same thing my self, how nice it would be to have a key bind to toggle gimballed to fixed. I was giving this some thought and had an idea that I think would work well and not require another key bind, and that is a slight change to how the target key works.
The key would work normally scrolling through available targets until you actively engage some thing in which that target becomes
"fixed", hitting the target key would now clear your primary target and revert the key to normal operation.
This would allow you to quickly drop a target and in so doing all gimballed guns would become fixed until you select a new target.