I love character discussion :D Let me see if some context helps out here...
Gina has a bad conscience for withholding information and keeping secrets from her friends already. She trusts Stella, since both fight back to back for their life in the Streets and she knows, that by not helping Stella anyway she can, she puts Stella at risk. With Stella confronting her, Gina is suddenly pressured for time, afraid to lose her only ally. The Doctor is in hiding, the magic shop bought out and she can't share the shakes, so any other option is uncertain.
Gina doesn't like using the potion, but she also knows the user can't take away strength by force. Stella cannot steal, just getting gifted other people's strength. And Gina never expected Stella to use it with such an efficiency. She believed this would be a minor boost to get her started.
Also the main goal wasn't only to make Stella stronger, but most importantly to drive away Jade into Xici's arms.