Look around. Gimbal guards have been associated with compass abnormalities (carbon fiber version), camera problems and other sorts of issues. DJI is well within it's rights to void a warranty on any modded product. That said, they can't possibly know unless you send them a picture or glue the thing on there.
I've avoided gimbal guards because they seem to create as much trouble as they save - certainly they can be helpful in some circumstances but YMMV. They clearly are no panacea.
My birds are fairly heavily modded (lights, antennas, trackers, air flow covers taped over). Yep, DJI might void a warranty claim but I will take that risk. What I won't do is mod a brand new bird. I will run it around for a while in stock configuration to make sure it is a keeper, then mess with it. Right now I'm looking for a used P3P to do some more radical stuff to (camera mods). I certainly don't expect DJI to warrant any of that.
And Apple has been pretty good about this sort of thing. They repaired my out of warranty motherboard on a 2011 17 inch MBP when it failed after three years despite replacing the superdrive with another hard drive, replacing the hard drive with an SSD and stuffing more than the stock amount of memory in it.
WHAT DO YOU WANT TO SPEND
Do not taunt happy fun ball.