Here's the answer from Avast Support.
"Hello!
Thank you for your email.
The Silent/gaming mode is designed specifically for the times when you want to use full-screen applications without being disturbed. It does not show any warnings whatsoever (but it keep protecting you and silently blocking any threat detected).
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Best regards,
Arina Zhdanova
Technical Support Specialist
Avast Software s.r.o
Budejovická 1518/13A
140 00 Prague, Czech Republic"