两者都有“获得”的意思：
gain 是普通用词，用法与 acquire 相似，常接抽象名词作其宾语，但不如 acquire 正式。 gain 可接双宾语，相当于 win、earn 的用法。
1) I am new in the job but I am already gaining some experience.
虽然这项工作我是新手，但我已经在获得一些经验。
2）Don't go back to your old eating habits, or you'll gain all that weight again.
别又恢复以前的饮食习惯，不然你又会变成跟以前一样胖。
3）It is imperative for a graduate to learn from the beginning to gain some practical skills.
对于一个毕业生来说，从头学开始学习获得一些实际技能很重要。
reap 本意是“收割”庄稼，可引申出“收获”，即“获得”的意思。
1) During past technological revolutions few of the companies that pioneered the
change proved to be good long-term investments, though some reaped the financial rewards.
在过去的科技革新中，没有几家率先进行技术革新公司证明是理想的长期投资者，尽管它们中有一些获得了经济回报。
2) Cultivating a wide circle of friends may reap a handsome payback in their
careers.
拥有一个广泛的朋友圈可在事业上获得一个丰厚的回报。
