Can someone ETS, get a GS job, and still retire at 20 years, like they would if they had stayed in the military?

You are able to but back your service and place it towards your Federal retirement. While it will credit you and enable to to retire at 20 if you choose the percentage of retirement is not comparable to the military. The majority of Feds, Non Law Enforcemnt and specialty fields draw only 1.1 percent. It is by any standard a good deal, in the private sector your military time would be a loss.

No, you won't draw a pension at 20 years like you would retiring from the military. There's other factors that go into it, but there is a minimum retirement age for federal retirement

I was a commander at DFAS-Indianapolis. I befriended a person who was in charge of an internship program at DFAS that led to GS-11 positions in a couple years and would put them on the fast track to GS-12. If they were good GS-13 was a real possibility. As the commander I took note of the finance NCOs who retired. If I admired them professionally and personally I told them to stop by the office.

I smiled and told them how impressed I was with them professionally and personally. I proceeded to tell them I was putting them into the internship program. They were stunned, floored, and thankful. I had the power to make one phone call that would transform a person’s life by giving them an opportunity they would otherwise have not gotten. I loved having that power, and I loved helping the male and female studs who worked for me. I told them I don't want anything in return except one thing. Help someone else.

Damn I was a nice person! Lol :)