Hi, Deanna. Yes, hyponatremia is listed as a side effect of Gleevec, though it's a rare side effect.
Here is the official prescribing info, from America's FDA: https://www.drugs.com/pro/gleevec.html#s-34084-4
From that document, hyponatremia seems to occur in less than 1% of patients on Gleevec. It also might possibly be from the blood disorder itself.
Are the doctors at the A&E or the GP's surgery saying it's not related? That would be odd.