Considering Rockstar Social Club codes were sold in every other store, I'd guess that the amount of non-Steam copies would be larger.
Even still, it's just a guess.
All of the other stores combined aren't as big as Steam. Even after a concentrated effort to drive sales to their own platform, and even with crazy before and near launch discounts of 40%+ on places like GMG.com, the Steam version of The Witcher 3 quickly outsold the GOG Galaxy version that everyone else offered.
Rockstar didn't actually make any special effort to drive sales away from Steam (they just wanted people using their social club online component), so my guess is that the ratio was much lower than The Witcher 3's.
At the very best, the PC version was the second worst selling version.