Is it common for games to get a anime?
Just like in this answer, let's take a look at /u/homu's data. 



This basically answers your question right here: between 8% and 15% (avg. 10%) of anime in each year from 2000-2013 were based on a video game of some variety. Games are a minor but not insignificant source for anime. 

Now, if you're asking "what percentage of video games get turned into an anime", the answer almost certainly rounds to 0%. There are a lot of video games out there (probably in the high hundreds per year, at least), but less than 200 anime per year, only a fraction of which are based on video games.

This data set only covers TV anime (thus excluding ONAs, OVAs, and films), but I see no reason to expect that an unusually large percentage of any of those media would be based on games.