The Arrow-verse has mostly shied away from canonically introducing a Green Lantern, aside from the tease at the end of the Arrow series finale, so there’s no established Earth-1 Alan Scott actor to potentially reprise a Green Lantern role for Stargirl.
John Wesley Shipp has played Jay Garrick on The Flash, which is now the Arrow-verse’s most senior ongoing series.
Stargirl could conceivably introduce an Earth-2 Garrick played by Shipp, which would likely be a draw for Arrow-verse fans to check out the DC Universe series.