Silent Gaming Mode is a way to disable alerts from stealing focus when you're in a game. Unless Avast is constantly popping up and telling you you have a virus, it is of no benefit to you.
That said, I've been using the free version of Avast! for probably about five years. I can't actually tell you if it's any good or not because ... I've never gotten a virus. (That's a good recommendation, in other words)
It's also way better than McAffee in terms of performance hit, so it's pretty good that way too. Good luck!
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