"Gain weight" 是增加体重的英文表达。
来源："Gain" 是动词，意思是 "增加"，"weight" 是名词，意思是 "体重"。
例句：
I need to gain weight before the weightpfting competition.
我需要在举重比赛前增加体重。
She's been trying to gain weight for months, but she's still too thin.
她已经试图增加体重几个月了，但她仍然太瘦。
He's been gaining weight since he started working from home.
他自从在家工作后一直在增加体重。