I got an aluminium GT Avalanche 3.0 and it has threaded holes on the top of the seat stays as you might expect, but no corresponding holes down towards the dropouts. It has mountings for a disc brake (not in use) and a replaceable dropout, both of which might serve as mounting points with a little jiggery-pokery. But other than a large amount of Galvobond (that metal strip stuff with holes in it) I'm a bit stumped how to attach a rack onto it. Old school rack that mounts on the axles? It's going touring and needs to take a fair bit of weight so I don't think seat-post mounted rack is the answer. Also the GT triple-triangle arrangement means that the holes on the seat stay are way lower than usual. Help appreciated, sorry there's no picture...

Most bike shops sell small rubber-coated clamps which you can wrap around your stays. I used two of these to mount a Blackburn Mountain Rack on my eyelet-less Schwinn KOM-10, and I mounted the one on the right immediately above the chain hanger, to prevent slippage. This arrangement has served me well for several years.