If it's a fiber gear you don't want to re-use it anyway. Those are wear items, like belts, and it's pointless to re-use 'em except in Very low mile situations.

So, if you're anywhere near as lazy as I am, you can just block the motor rather than holding the cam gear. Obviously you don't jam the teeth of the gears themselves. If you haven't got a flywheel stop tool, just stick the car in 4th, chock the wheels, brakes on, long cheater bar, you'll get it done.

Since you're already replacing the gear, don't worry about torquing against it. Have at. The gear to be careful with is the new one. And, that new gear will survive one careful torquing on installation. Really, it's okay.

I would agree that a standard handheld torque wrench is the safest way to tighten the new gear: nobody ever broke the cam nose off when they carefully did it by hand. The standard torque applied slowly by hand will do No Damage Whatsoever to the gear teeth, despite how scary it may seem.

You should make a tool to restrain the cam gear, similar to the SVO tool. I made mine out of heavy section angle iron, and welded two 5/8 inch diameter pins on it to engage two of the three holes in the gear. The angle iron is just over two feet long. With that installed, the nut can be removed/tightened. It's possible to leave the crank and cam gear meshed, and then block a crank counterweight to allow the nut to be undone or tightened, but that's only an option with steel timing gears. With the fibre ones it's not a good idea to impart the torque needed through the two or three teeth that are engaged in the crank gear.
Removal with an impact gun is ok, but to install that way....you better be good or the cam snout can be broken off.