@Marc70: I think ATTI mode only use the gyroscope... but if you trigger RTH failsafe, it'll use GPS to find home point whatever mode you were in before.
GPS will be more prone to atmospheric drift due to rapid changes in the ionosphere. I am not sure how much a G2 level event will actually be noticeable but the likelihood of drift is higher. IMU should not be effected by a G2 level event.