Hi Unregistered
You used the wrong word in your post. "Had been practicing" is the past perfect continuous form of the verb practice, and in this case practicing is not a gerund. Practicing is used as a present participle. I presume you wanted to know when to use the past perfect continuous.
Basically, the use of the past perfect continuous is similar to the use of the past perfect simple. You can use it to establish that one past activity happened before another past activity. The use of the past perfect continuous in your sentence suggests that one past activity ("got into university") interrupted another previously on-going/regular activity ("had been practicing").