Only way at this point would be paying Garmin to replace it (in short, replace the whole watch. If you get a glass screen protector for a Forerunner 935, it will cover the glass perfectly, and may hide the scratch as well..
you may have to use some diamond polishing paste and very fine sand paper with water but I would not do it myself as it is expensive watch. Take it to a good watchmaker as they may have proper tools to polish it quicker as hand polishing will take hours and you may not see the result. Also you may see some slight imperfections on the glass afterwards.
There are some liquid screen protectors that heal small scratches. You put them on and let it get dry... Try one of these... Search in Google as liquid healing screen protectors
After getting a scratch almost exactly like that on my old Vivoactive 3 gorilla glass, only about a month after using it, I think the very first thing I would do is buy a screen protector for any future gorilla glass watch. It's really not very scratch resistant. My new 6X is sapphire, so I don't really have to worry about it, but if I got a gorilla glass version I'd for sure put a screen protector on it ASAP.
I'd clean the glass (and the scratch) as much as possible with alcohol, and then apply a screen protector over it and hopefully it mostly hides the scratch and doesn't cause an ugly air bubble.
I would NOT try polishing it out - that scratch looks deep, and if you polish it until the scratch is gone, the glass will have develop a wavy contour and screen protectors will never work right after that.
My opinion is to try simpler solutions rather than try the most risky ones that may cause more damage to the screen. Even if it won't fix it 100%, it may become minor and hard to notice. Then apply a screen protector just to ensure no further damage is done to your screen
