Gainly can be an adjective that means graceful. It can also be used as an adverb that means very or completely.
Originating in Middle English, the word gain has many meanings. Among other things it can mean ‘to win.’ You can think of gainly as a synonym of ‘winning’ as in, a ‘winning personality.’
Sheila’s gainly demeanor will get her far in life. Everyone loves to be in the company of someone charming and sweet.