gain in (something)
To achieve more of some thing or attribute. Even just working there a year, I gained in knowledge of how the industry really works. That timid little girl has certainly gained in confidence over the course of the school year.
gain in something
to advance in a particular quality. Todd grew in stature and gained in wisdom. Mary gained in experience as the weeks went on.
gain in
v. To increase with respect to something: The athlete gained in strength, but lost in agility.