I dunno anything about real steel p90's but I have that DTP rail you're refering to on my tm p90, just took a pic today with my new laser
now, I have a GB tech metal receiver as well. when installing the dtp rail i found it was extremely tight. i had to sort of tap it in with a hammer and cloth. I'm not sure how different the gbtech is from the real steel one, but you can be sure it'll be a tight fit.
the rail DOES come with screws. it doesn't use the original p90 ones due to the fact that the metal is pretty thick and the screws need to be longer. they fit perfectly however after some hammer convincing and its solid as a rock. Extremely happy with the rail/gbtech receiver combo, but one thing to note is the rail paint job seems to be a tad shoddy. dont get me wrong, it looks fine, but the paint coating seems to be rather thick about .65mm which makes everything tight and sometimes hard to fit, and it 'chips' rather easily. A few pieces of paint chipped off when taking my aimpoint on and off. Also, the arms mount for the laser was very tight, I dont think i'll be able to quick release it anytime soon. Other than that, its a good product, it works exactly like how it looks like its suppose to. I got mine from UNcompany. Oh, and the side rails are removeable as well. it's held on my 3 allen screws so you can take either side off/on if you need.
g'luck with your p90.