There is no consistent difference as all of G&G's lasts fit the same. EG's on the other hand were designed by a number of different people over the years and fit differently. For example, the EG 202 is half a size larger than the EG 82. Specifying the G&G someone would need would require him to specify the EG he was comparing.
Tony Gaziano designed the 888 & 82 lasts before forming leaving to form his own company and make his firm's lasts. They can be seen as prototypes to what he did with G&G's lasts...including increasing the visible angle of chisel. Tony mentioned his firm's lasts are a bit roomier in the toe box...but very similar in sizing and width. My own personal experience with the 888 compared to the TG73/MH71 confirms the above and suggest if you own any EG 888 or 82 shoes, consider the same size equivalent from any of G&G's offerings.
taking this into account, when comparing G&G size to EG are you supposed to match to the UK size of the EG or the U.S. size? For Ex. Would a 7/7.5 EG match to a 7 or 7.5 G&G? Has anybody found this changes when dealing with an Oxford vs a Derby because of the open flaps?
WesBourne- The G&G's RTW do come in a D width. It is there narrow width.
Word. I believe the suede RLPL Narvell monks (as well as other G&G made RL shoes) are made in a D width. I didn't know their own RTW was available in D width as well. Fwiw, Bespoke England doesn't list the D width. Also, iirc, most reported that the Narvells fit 1/2 smaller than their listed size.
If helpful for anyone, I wear 10.5UK/11US in the EG 888 but 10UK in G&G TG73 and MH lasts. The G&G's are snugger. I cannot wear a 10UK/10.5 US in the 888. Not sure how much bigger 10.5 UK in the two G&Gs lasts would fit. It would probably be wearable.