I think it was the mentioning of Gap that probably put people over the edge as it's a brand!
Honestly OP, I feel your pain. I have the same problem with my mother and my MIL. I can't give a rat's posterior where the clothes are from (so long as there's a chance they HAVEN'T been made by a two year old in a sweatshop) but I do mind when they buy either random items for them that don't go with anything else in their wardrobes so don't get worn (and I'm not talking matchy matchy here) or things that I just know they won't wear. So MIL for instance, bought something for DD1 (aged 5) recently that is a very nice item of clothing but a) is a colour that makes her look like death warmed up so she'll never wear it and doesn't have anything to wear it with b) doesn't suit her body shape (bless her, she has her mother's total lack of an arse) and c) she doesn't like!
Waste. Of. Money.
And I can't even put it away for DD2 as there isn't a hope in Hades that, unless that child has a major personality transformation in the next 18 months, she'd even allow it into her room!
My own mother I can be a bit more forthright with. She bought a bunch of stuff and showed it to me on FaceTime recently. I went through it saying yes, no, yes, no and I'd say 40% of it was returned. She kept holding things up to the phone and looking at me pleadingly saying "But I think it's gorgeous." You may well do mother, but I'm telling you right now, they won't wear it!
But the punchline of that rant is, there's very little you can do about it. See what the children like, and bring that, don't waste suitcase space on the rest, and return what you can, or donate it to a children's centre.