You talk about watching Gina practicing yoga but I think you're really talking about Katia, who you have to buy a new dress for sports. To find the place where to buy it, talk to Jessy on Friday afternoon when she is in her room, she will tell you.
On Saturday Afternoon, Gina is in the Lounge practicing Yoga. The Quest is Gina's. It's under "Get interested in her hobbies" - Watch how Gina practices Yoga. I've checked it several times and everything is done.
The tasks are
Spy on Gina 3 times (I've been there every Saturday and Looked at her in multiple fashions... 3x [Look/Look more/Look more], 3x [Look/Talk/Leave], 3x [Look/Talk/Talk]... Under the [Look/Talk/Talk] efforts in the second [Talk] at the end of the discussion she offers to teach you yoga. It comes up on the screen that you didn't bring a sweatsuit and that you'll need to get one. I figured since you had to get sportswear for the Katia quest, it was related. But it isn't I'm now finished with Katia's quests)
Affection: >= 30 (it's Green - I have 97)
Sexuality: >= 30 (it's Green - I have 46)
Location: Lounge
Day: Saturday (it's Green - I'm on that day right now)
Time: Afternoon
I figured the Spy on Gina 3 times was related to the night option of giving her a massage. When you get to a point she changes and you have the option to Spy on her. I've done both those options - Get Closer (3 times) / Spy On Her (4 times). That massage quest is completed.
Still nothing.
EDIT: Mother puss-bucket... I missed 1 [Look][Keep Looking][Keep Looking]... nice Gina masturbation scene...