IMHO,
The Old Growth Classic is a really good medium for continuing to examine the boundary between dropbar gravel/CX and XC mtb. Wish I coulda gone. The pros all race in the drops, but I'd guess the entire event is 50/50?
I used to mtb all over that area covered by the course, and changed my gearing so that I could commute to the trailheads at a reasonable pace. XTR had FD 48t clearance and I ran that with an XO setup whatever year that came out. The end result was 48/34/24 11-34 I think.
I'd try GRX on a CX with a 52 flat bar.