I think Grace made more from her sponsorships than what Gymshark pays, but we'll never know for sure because no one talks about how much they're actually paid. She definitely made a lot of money in a very short amount of time, but I do wonder exactly how much if she's managed to get a mortgage based on her income and the bank thinks she's good for almost £2million. That would mean if we take the triple income that mortgage applications are usually based on, she was earning around £600k a year at least for more than one year because self employed people have to prove a consistent income.
I've heard some other younger youtubers talk about how much they get paid for deals before (I think it was Kate Elisabeth or Olivia Neill maybe?), saying they have been paid £15k for a contract with a clothing brand eg Nasty Gal for a few posts over the course of 3 months.
Bearing in mind these girls at the time (they said this months ago when they were only on about 150-200k subs) had a much smaller following than Grace did at the time, and Grace did sooo many sponsored posts with fast fashion brands like these over the 3 years she was at uni, on top of Gymshark, other ads in videos etc... I don't think it's too far fetched to assume that she did manage to make the £360k she needed for her deposit.
Also, her outgoings would have been so much less than most other people considering the amount of stuff she gets gifted.