Since Hank turns down the offer to return to El Paso, Steve Gomez is promoted instead of him. However, later in the series, in seasons 4 and 5, Steve Gomez is still in ABQ, working with Hank.
Why is he not in El Paso?
Steve Gomez returned when Hank got attacked by Leonel and Marco Salamanca (Tuco's cousins). And Juan Bolsa also got shot during phone conversation to Gus by Mexican federals during investigation of Hank's attack. So after the death of Juan Bolsa, Leonel and Marco Salamanca whom DEA is searching in El Paso, there is no requirement left for Steve Gomez to investigate in there, so he doesn't go back after Hank's attack. Because the case was solved.
If I remember correctly, he accepted the job. Maybe he moved to El Paso and came back from time to time? Maybe he came back to help Hank when he got Jessie.
The plot isn't really clear on that point