GXP effectively does not have adjustable preload. GXP is sort of a crappy design where the bearings are loaded unevenly. I think the cap you're talking about is the extraction bolt which is left in place and allows the 8mm headed bolt to self extract. You can usually use a normal 8mm wrench to remove a GXP crank-sometimes a breaker bar is helpful. Honestly you probably don't need a torque wrench to reinstall one--it's pretty darn tight, and you're very unlikely to have consequences from slightly overtorquing. Try removing the crank and see how the bearings feel--it could just be they're worn. All external bearing systems are fairly sensitive to alignment, so assuming you have a threaded BB shell you may get better performance if the shell is faced. Lastly, there's a lot more sealing on a modern BB like that, and you should not expect it to spin freely in the stand like 70s neuvo record. The seals create comparatively a lot of friction unloaded, but the loaded efficiency remains good (though, admittedly, worse for GXP than most systems). If it seems like I'm trashing GXP, I am, but I also have two bikes running GXP that are fine and I don't worry about it.