A GRB close enough to fry Earth would also fry all the other planets in the solar system. Mars won’t help.
Mars however is very cold, highly radioactive, practically no atmosphere, and vulnerable to meteor strikes. So to fix all those problems inhabitants would likely be under 1 meter to minimize air leakage, radiation, heat leakage, etc. So it might well be that 99% of people survive a 1M deep sterilization ... only if self sufficient, which does seem a stretch, at least for the next century.
One example is infrared which we sense as warmth. I am somewhat fuzzy here, but I imagine that these photons bump into the atoms of our skin such that they tend to shake more vigourously than before. This is heat.
I can imagine that gamma rays of a gamma ray burst are plentiful and hard enough to influence the tardigrade matter to an extent to even kill tardigrades.
This said I see a way how tardigrades could survive a gamma ray burst. They are inside an asteroid or even inside a Mars-like planet. Even if the gamma rays ablate a significant chunk of the crust, enough will remain to protect the tardigrades.
So some tardigrades might be lucky to survive a gamma ray burst nearby.
Just speculating here.