I used to but now that I have recently found out it is sinful, will Allah forgive me?
Only Allah knows. Remember Allah is the most merciful and if you truly repent having done it, you need not worry anymore.
If you did not know then it is not a sin and you will be forgiven. However it is must for you to learn about things, know things, understand them and now that you know you should never do it again. Ask for forgiveness from Allah, He is the most Merciful. Allah knows best.
Indeed Allah loves the penitent and He loves those who keep clean (2:222)
Do they not know that it is Allah who accepts the repentance of His servants and receives the charities and that is Allah who is the All-clement the All-merciful? (9:104)
If you did not know because of being careless about your Islamic duties (in other word, you could've learned your duties through different ways like asking or reading or .... but you neglect to do so) you have committed a sin but if you did not know about this rule in Islam and did not even have a hunch that there might've been some restrictions, you did not commit any sin.
However if you fall in the first category, the way of repentance is always open. If you are capable of apologizing those of whom you used to talk ill, without getting into bigger trouble. you should do that and if not, just ask Allah to forgive their sins.
However the first and the most important step of repentance is a strong decision not to come back to sins at all.