You wouldn't get hired at GS-15 as a new hire. You might reach that after 20-30 years, and you'll need to get a Ph.D.

Also, DC Metro is an extremely high CoL area (not SF levels but probably around Seattle levels). Normal GS-15 pay is $112k-146k and you'd trend towards the bottom of that in most areas.

So, $112k average at the peak of your career, with a required Ph.D. More realistically as a new hire you might be GS-12 (which only takes a master's) which is $68k-89k, so you make $68k in most areas. That's certainly not great as far as competitiveness with tech salaries, that's pretty close to the bottom of the market these days for a new hire working entirely remote, not even highly-desirable talent.