Well, so there's this:
http://journals.sagepub.com/doi/abs/10.1177/082957359601100215which finds that, in her small study population, the OLSAT underidentified GT students at the 98, 97, and 96th %ile cutoffs (using a comprehensive WISC measure as the standard of comparison). Correlation of .39 at the 98th %ile or higher between the two measures, which isn't all that great.
And also, the SEM of the OLSAT-8 is just under 6, which means that your DC's score is within a SEM of the cutoff score. (That's on the SAI, btw, not the scaled score, so actually the relevant SEM is likely even larger in this case.) So there's that.
I would start, however, by considering whether the GATE program provides sufficient support to be worth the advocacy.
