Taubah and Repentance

Taubah for making gheebat

Q: I've made gheebat of many people but now I have repented.

1. Is it necessary to ask them for forgiveness as they do not know that I've made gheebat about them?

2. If I've made gheebat about somebody many years ago and they knew about it at that time but most probably forgot it now, do I have to ask them for forgiveness?

Repenting for drinking beer

Q: I drank 2 small sips of beer that contains 6 or 8 % of alcohol but I didn’t feel any type of difference in my body. Everything was normal and no intoxication and nothing different in my behaviour.

My question is, can I go for payers to the mosque or I’m not allowed to enter the mosque for 40 days?