I have the same problem. i can't have acces extranonce with my gridseed.
I use minerd,and ccminer 1.5.39 per my gpu.
With gpu no problem,i have extranonce access
Use BFGMiner for gridseed and use #xnsub after pool:port, example : stratum+tcp://stratum.nicehash.com:3333/#xnsub#skipcbcheck
-MZ