put on weight 与 gain weigh 各有两个意思：
1.加重 （对物体）
2.胖了 （对人和对动物）
就第二个意思来说,两者没有本质上的区别,通常可以互换,例句：
Not having seen her for a few months,I was shocked to see that Jane has put on so much weight.
On seeing her for the first time in several months,I notcied Jans has gained a lot of weight.
也希望点击“选为满意答案”及时采纳,