The most common and most likely answer to this crossword clue is the 6 letter word MASSED
I've seen this crossword clue in The Sunday Telegraph
M
A
S
S
E
D
Buddy explains!

The straight part
We know that "Gathered" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gathered = massed.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 MASSED mean?
MASSED verb- To form or collect into a mass; to form into a collective body; to bring together into masses; to assemble.
- To have a certain mass.
- To celebrate mass.