Gang can be used as a slang word to mean a group of people most likely familiar to you like your friends. In this case, the group of people that work in the office. A treat is anything that is given to someone for entertainment or to make them happy in someway. Like you give candy to a baby, you are giving them a treat. It makes them happy. Affection is just a positive or warm loving feeling you have for someone else. So the speaker is saying he wants to give his group of friends at the office something to make them happy so they will like him again.

Treat in this sentence means a small gift. Gang is a word that means a group of people. It can have many different meanings, but in this case, a gang is a group of friends. So the character is saying they will give the group of people a small gift, so that they will like the person. Using the word treat is totally normal. Gang isn't a weird word, but not common.

Can i practice my english here??