The thin capitalization rules as currently drafted don't apply to trusts, so there'd be nothing for GAAR to apply to. GAAR is nothing more than another section in the Income Tax Act, and its application is in the context of the Income Tax Act and whether or not there's an abuse of the Income Tax Act.
The thin cap rules are drafted and specifically do not apply to trusts. So in my view, GAAR would not apply to impose the application of the thin cap rules to existing income funds, as currently drafted.
If an amendment to the thin cap rules were made, then it's a different story. But as currently drafted, there's nothing for GAAR to apply to.