Should I mention that I worked as a professional online poker player on my university application? I'm 27 years old and I'm applying to university. On the applications it ask the student to list activities and employment during time away from school. I played online poker for living for ≈4 years, should I mention this? Or would I be looked down upon and perceived as a gambling addict? There's no room on the application to go into details.

If I don't mention it then I'll have this big empty gap with no jobs listed – only activities. I'm applying for undergraduate studies into CS.
I like @DanRomik's analysis, but the same analysis leads me to a different conclusion: yes, list it. 

First, it gives a better picture of who you are and what you have been doing.  

Second, while some people may be prejudiced against poker players, in most applications there are things (and lack of things) that may be viewed positively by some committee members and negatively by other committee members.  I don't think the concern should be too great that it will lead to too much veto-ing of your application, as might be the case for being incarcerated for a violent crime, say.  (Of course this is my personal opinion and not based on any hard data from surveys--I imagine it might be an issue at some schools, but probably not most.)  I personally would find lack of activity for 4 years more of a concern.

Third, the fact that you are no longer playing poker professionally (I infer this from your use of past tense) indicates that, even if you were addicted to gambling at some point, you overcame it.