Years of internetting have taught me this:
An individual with the word gamer in his/her username will more often than not be obnoxious, loud and uninformed.
Most people that enjoy playing computer games are of course nice and balanced individuals.
I think it is mainly a matter of the persons age. Most persons that identifies themselves as "gamers" strongly enough to let it reflect in their choice of user names are probably children - and children are not seldom obnoxious, loud and uninformed.
As a person grow older, their identities grow more broad and less emotional, I think. So, more mature people do not as often as younger have user names that is focused on a singular hobby or interest. That is, at least, my theory.
I did identify the "law" while reading very balanced and informed comment on YouTube, by a person with "gamer" in his/her name. The fact that the comment was balanced and informed surprised me. Which in turn made me think about the reason that this surprised me.