Student or Learner
A character is named Garth the Gimble. Is this Gimble something in English. I'm not able to find the word at any dictionary
I think the word "gimble" was invented by Edward Lear the humorist. In the poem Jabberwocky he has: "'twas brillig and the slithy toves did gyre and gimble in the wabe. It is a nonsense poem containing several nonsense words.
(Not a teacher)
If we are taking it to be 'gimble' from the 'Jabberwocky' poem from 'Through the Looking Glass', then Humpty Dumpty tells us what it is:
Note that it is a nonsense word.Alice: 'And what's the "gyre" and to "gimble"?'
Humpty Dumpty: 'To "gyre" is to go round and round like a gyroscope. To "gimble" is to make holes like a gimlet.'