Heat will increase loss of atmosphere, but vaporized isn't the right word.
The issue would not be losing even 25% of the atmosphere instead of close to 0.00% per year, but that boiling oceans, steam, and air will hit say 1000F on the size of the GRB and then over the next hours/days mix with the rest and end up with a average of 500F.
Even with that, I suspect many but not all species of life would be extinct. Anything that lives in the deep ocean might be mostly unimpacted.
The issue would not be losing even 25% of the atmosphere instead of close to 0.00% per year, but that boiling oceans, steam, and air will hit say 1000F on the size of the GRB and then over the next hours/days mix with the rest and end up with a average of 500F.
Even with that, I suspect many but not all species of life would be extinct. Anything that lives in the deep ocean might be mostly unimpacted.