GMATPrep's standard deviation hasn't been published but the official GMAT has a standard deviation of 30 points, so GMATPrep's SD will not be smaller than that. It's likely to be either the same or slightly larger, depending upon what differences there are between GMATPrep and the real test.

We do know both use the same algorithm, but we don't know whether GMATPrep uses a wide enough pool of problems to maintain the same standard deviation. I always think of it as 30-40 points myself.

So, congrats! That's a great score to get. You did do the essays also, right? I ask because you wanted to know how great of a predictor it is, and the answer depends upon whether you took the test under exact test conditions - essays first, no more than 10 min. breaks between sections, etc.

So, assuming you took it under normal test conditions, the SD tells us that you have a 2/3 chance of scoring within 30-40 points of your score (750) and a 1/3 chance of scoring outside of that range. (This is also what the scores on the official test tell us - although, there, the SD is about 30 points. Most people are surprised to find that the test is not anywhere near as precise as most assume it is - it is NOT precise to within 10 points, even though that's how small they make their scoring increments.)