In particular, they are descendents of younger sons of the nobility. Gentle has to do with land owning, not good manners, though manners may be considered a mark of gentility. Money also has little to do with gentle birth. This sometimes applies to nobles, though it is not fair to say that any merchant has more money than any noble. The term middle class is unknown in period.
People are much more specific about their place in society.
When yeomen get a little money, they tend to buy land, which makes them landowners. If the family are provident and continue to acquire and hold the land for at least three generations, they will come to be counted among the gentry. Craftsmen or tradesmen may be considered equivalent to the yeoman class. Town people of any rank consider themselves superior to country people of the same rank. Peasants are tenants who work on someone else's land for wages.