Although bearings are often crap in many makes of BB, I’d be amazed if there were ever manufacturing defects in BB cup threads. I’d be more concerned about your frame’s BB threads than anything else.
To answer your question, there’s hardly any difference in thread ‘looseness’ between one make and another (amount of thread-lock applied in factory probably makes more difference). They would never compromise on something that critical, and it’s not particulary difficult for Shimano/SRAM and others to create good quality threads of the same tolerance. Good bearings are much harder to make !
In normal circumstances, the only reason one model of BB cup would be harder to install than another would be the length/depth of threaded section. For example, RaceFace DH cups have longer threads than normal BBs, and sometimes the threads in a frame aren’t as clean that far inwards. But that’s never a problem as once it’s screwed in that far, you just need to turn it a bit harder. However, you should never need to apply any force with a spanner until the very last moment.
Did you buy and fit your GXP BB new? If not, perhaps a previous owner bodged it…and damaged the threads.
And sorry to ask, but are you sure you’re screwing the drive-side cup into the correct side ?
As for bearing life, as mentioned already, if you want a BB that lasts with GXP cranks, get a Hope. You’re correct in that SRAM don’t seem to sell a ‘normal’ steel bearing BB for much more than £20 or £30 – and for that money you do indeed get bearings relative to that low cost, i.e. they’ll last about 6 months.