Gargan, 32, joined the Galaxy ahead of last season and then compiled MLS career highs in appearances (29), starts (27) and assists (five). Terms of his new deal were not disclosed.
In the SuperDraft, the Galaxy said it chose Louisville defender Daniel Keller in the third round (62nd overall) and Penn State goalkeeper Andrew Wolverton in the fourth round (82nd overall).
Keller, 22, started all 22 of Louisville's games as the team's captain last year and played in a total of 74 games during his four years at the school.
Wolverton, 21, had a 38-23-8 record as a starter during his four years at Penn State while recording 225 saves and 32 shutouts.
The Galaxy last week selected forward Ignacio Maganto of Iona in the first round of the draft (21st overall). The club did not have a second-round selection.