The most common and most likely answer to this crossword clue is the 10 letter word ASSEMBLING
I've seen this crossword clue in The Daily Telegraph
A
S
S
E
M
B
L
I
N
G
More clues leading to the result ASSEMBLING
Buddy explains!

The straight part
We know that "Gathering" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gathering = assembling.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.
What does ASSEMBLING mean?
ASSEMBLING verb- To put together.
- To gather as a group.
- To translate from assembly language to machine code
- A gathering or assembly.