25% seems kinda low from my perspective, which isn't saying much tho
I've seen several GMCs in the wild and on here, not much less than Chevys.. I would guess the ratio closer to the Chevy numbers..
my logic, for whatever that's worth either

is that there seems to be a higher ratio of 3/4t to 1/2t in the GMC line as opposed to Chevy and the Longhorns/GMC 133" WB are all 3/4 or 1t..
Chevy 1/2t seems to dilute the overall total number of trucks considerably..
just my swag