Haven't rewatched, but it didn't seem remotely calculating how Gemma said it. It was from a high angle above the group so wasn't like a staged chat, but a more natural group convo that had some momentum and she didn't have her usual guard up, just got chatting with the rest of them, like any 19yo with her mates, and when the topic came up - think someone else said it initially - she just ran with it. Sounded like she thought it'd be funny, but hadn't thought it through and like everyone else was shocked at the horrid turn from Jacques.
The idea she was deliberately pushing his buttons to get that OTT reaction feels way off to me. There's definitely a northern sense of humour of taking the piss/winding people up that she and Jacques would both indulge in, so this could well have felt like that to her and he should have been able to take it, god knows he dishes it out. To tell someone they've been called immature is the perfect chance for that person to prove they're not immature by handling it well, laughing it off, and pulling Adam for a chat if he had an issue with it. But in any case, I really don't think it was a strategy and Gemma could not have predicted his explosion, even if she already knew (and had kept quiet) about his flaws.