Gap's quality is higher than H&M, IMHO. Zara is another of Gap's rivals that seems to be expanding in the UK highstreet, although I rarely find anything worth purchasing.
Zara always has queues when I see it, but my wife tells me the quality of everything she's bought from there has been dismally poor, especially given their relatively high prices. Usually the issues she's described are around the quality of the work and not the material, seams falling apart, buttons and accessories falling off etc.
Inditex (Zara) is a juggernaut compared with Gap or Primark. The differences in scale of operations, targeting, and branding are massive. It is like comparing AWS with Cloudera.
I find the rapid store expansion a problem and wouldn't be surprised if they repeat the same mistake, although they haven't plastered their clothes with Zara logos.
Uniqlo is the only store that hasn't yet repeated this mistake.