Well zas240, keeping in mind that you fully understand this list may not, and likely will not, win heaps of games; lets see what can be said to help you out without taking to much from the feel of this army.
Now first things first, I went and did the numbers and your actually totaled at 1514, so your gonna have to lose something along the way.
Grey Knight Grand Master, Mastercrafted Nemesis Wepon, = about= 180
And heres where we can free up those overspent points. Rerolling a hit is nice, but does he honestly need the mastercrafted weapon? I suggest removing it that way your under again.
Grey Knights (9), Justacar, Psycannon Bolts. =about= 440
Grey Knights (5), =about= 590
Alright so everyone's pointed it out to you, fifteen marines, and one is at five strong. Not gonna last very long in any game, twenty terminators or not. So you have these fifteen, make the five man a little more survivable; take two out of the ten man and run an eight and seven man squad. Eight is still a decent size and seven is just a single body short of an eight man squad.
Terminator Command Squad (9) Incinerator. =about= 1004
Terminator Squad (10) Incinerator.=about= 1494
Not much to say about these squads; twenty terminators is a scary sight though and will atract a great deal of your opponents attention. And rightly so for that matter, twenty terminators aren't exactly easy to kill with average guns. This'll give your normal grey knights a little measure of protection.
All in all, your army lacks heavy anti tank, but can deal with most things it charges and many things that charge it. Some armies that don't bring a ton of high strength, low ap weapons (and such lists do exist, my Dark Eldar have seen that a almost a dozen times now) will be hard pressed to kill enough of your army before you really close the gap and can hit home with the terminators.