gain : 侧重指经过努力或有意识行动而取得某种成就或获得某种利益或好处。

If you gain something, you obtain it, especially after a lot of hard work or effort.

They realise that passing exams is no longer enough to gain a place at university...

他们认识到现在仅仅是考试通过，并不能确保进入大学。

get : 普通用词，使用广泛，可指以任何方式得到某物，也不一定要经过努力。

If you get something, you receive it or are given it.

I'm getting a bike for my birthday...

我过生日时会得到一辆自行车。