I just spent most of the day looking for data on Linux gaming, thought I'd share the results:
About Linux sales:
I used data that was posted on gamingonlinux.com and comes from the developers. The average share of Linux sales is (3.2 +/- 0.4)%. The median is 2.7%.
For Humble Bundle sales, Cheese has a very nice
collection of Humble Bundle sales data from May 2010 to February 2016. According to that, Linux accounts for 4.7% of purchases and 6.9% of payments.
It would be nice to have similar data for some bigger releases, but then usually bigger games tend to be very late on Linux, so it wouldn't be representative.
I also looked at Steam users and Steam Linux games:
I plotted the total number of active Steam accounts; starting Jan 2013 1% of that are Linux users according to the survey. So as of last February, that's 1.25 million. I tried to extrapolate and came up with 160 - 190 million total Steam users, though of course the growth has to slow down at some point. Still, considering that SteamOS/BPM isn't even counted in the survey, I think >1.5 million Linux users on Steam seems realistic.
Also, it's nice to see that the number of Linux games on Steam is growing at roughly the same rate.
As for other stores, according to the
GoL survey retailer statistics, about 90% of participants buy games on Steam, so about 10% of Linux users are anyway not in the Steam statistics. This, too, has stayed pretty much constant since September 2014, while the number of participant has an upward trend. It seems that Linux outside Steam has grown at roughly the same rate as on Steam, though the GOL survey has limited statistics with a lot of the same folks participating every month.
All in all, this brings the total number of Linux gamers to an estimated 1.6 - 2 million.