Garth and Kaldur's boyfriend are two distinct characters.
Garth was on the Team as Tempest.Season 3 has an episode where Donna is explicitly said to have retired, so she's only helping out because the shit Child is pulling needs as much help as they can get, Garth could be in a similar position where he's retired (If he ever became a hero to begin with), but helps out in crisis that are too big.