Apple were apparently able to consider both Jersey and the Isle of Man (both British Crown Dependencies) for substantially reducing their tax exposure, so it's presumably questionable, but this is the first I've heard of GAAR. The UK Parliament has the ability to create primary legislation on any matter it wishes in these regions, even if by convention it rarely does, and Jersey and the Isle of Man have had arguably justified reputations as tax havens for a long time in the UK.

One only needs to look at the other leaks in this dump to see the attraction of these locations (Formula 1 driver Lewis Hamilton stashing his private jet on the Isle of Man to avoid a substantial VAT bill etc).