Garry's Mod - Why don't i have the One Month achievement?

Question

I've played gmod for 1199.9 hrs in total and i have't got the achievement? whats wrong and yes its a long time 1199.9 hours although i hardly play gmod now but i played it for like 1 year without doing anything else like gcses so yeaaaaaaah im proud becuase i own that my playtime is that high!!