As I said earlier, The Oxford English Dictionary defines gentle - not maggot - in this sense as (quote) "the larva of the flesh-fly or bluebottle".
The word may have been around in speech for some time before then, but It is first recorded thus in print in 1578. At that point, Shakespeare was 14!
(I don't recall a character with the name Gentle in any of his plays, but - as I have not read them all - there may well have been. If I get the time later, I may skim through all his dramatis personae to check. Presumably, if he existed, he will have been a wriggly, rather unpleasant character.)