The end of my eight gates ajar, all opening (5)
I believe the answer is:
omega
'the end' is the definition.
(last letter of the Greek alphabet)
'of my eight gates ajar all opening' is the wordplay.
'all opening' suggests taking the first letters.
The first letters of 'of my eight gates ajar' is 'OMEGA'.
(Other definitions for omega that I've seen before include "Traditional story" , "completes a series" , "Greek finalist" , "Conclusion" , "Greek long 'o'" .)