The most common and most likely answer to this crossword clue is the 3 plus 2 plus 5 letter word GET UP STEAM
I've seen this crossword clue in The Daily Telegraph
G
E
T
U
P
S
T
E
A
M
Buddy explains!
The straight part
We know that "gradually gain more impetus" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gradually gain more impetus = get up steam.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.