I had a look into a dictionary to find out something about this verb, however, I am not that sure I will be able to use the verb actively.
ad verb definition 2):
She was steadily gaining in confidence. <= is it possible to say the same sentence without "in"?
Basketball is not a major sport in the Czech Republic, although it is gaining in popularity. <= can I both say "it is gaining popularity"?
Both ways are OK.