Does Halflife 2 run on Ubuntu?
Add your CD-key to your Steam account and you will get access to the game on all platforms for free.
TrailRider adds:
You will just need to download the Linux version. This is assuming that the OP has a Steam key with the disk (hard to tell the way the question is phrased) If you use the same steam account on every system, you just need to click the install button to start the download.
You can run halflife 2 using wine, not need Steam.
I just test it a few days before in 16.04. It works well.