Generally speaking Linux can be a gaming platform when you have an ecosystem with companies, manufacturers competing for customers utilizing a platform.
It's already happened. Yes, my friends, Android is that platform.
In the end without a market (with customers and demand) you not gonna have the manufacturers (contributing to the platform with device drivers) and the software companies.
So what is the target audience for Ubuntu Linux?
It is an operating system for personal computers competing primarily with Microsoft Windows. Personal computers are competing with the game consoles, also nowadays with the touchscreen mobile devices.
You have a lot of competition and Ubuntu doesn't really offer as a gaming platform anything besides it is free as beer (and for philosophers it is free as speech).
You get nothing for nothing, it's a vicious circle, without customers no developers.
You have to show some initiative or at least the software companies has to.
Valve just did that.
Being ready from a technical standpoint is vital, but not nearly enough.
I personally think that Ubuntu Linux should not be a gaming platform, we already have that from the Microsoft and Sony.
A maintained (and supported) Linux distribution is more important for servers and workstations.