Obviously I agree re: Charlize Theron, and she’s a fandom fave casting for Gabrielle, but Viva Bianca is a choice I had not considered! She was gr9 in Spartacus. 

^Very Gabrielle here.

It would probably be easier to age her up for the duration of time that she needs to be older, than it would be to make the older actresses you mentioned appear younger for the vampirism part. Also, as much as I love Meryl Streep and Rene Russo, they are both much stronger of facial feature than I headcanon Gabrielle, who’s meant to have kitten-ish features and resemble Lestat.