Social has a wider range of meanings, it can mean:
1. To enjoy the company of other people (e.g. I am a social person)
2. It can refer to events that are quite formal or of a sociable standing (e.g. I am going to this social event)
3. It can relate to the study of people in society (e.g. The social status of the working class)
Gregarious does have a similar meaning to sociable, but what I would say is that the word 'gregarious' is not often used in conversational English; it is more of an academic word (at least in my opinion).