I personally think that Gai would be extremely powerful as edo if he could use 8 gates since his body and chakra would be constantly regenerating so he could use the gates without the time limit and consequences.
If he is in 8 gates mode then he is deffo a big threat. But then again being of konohas most powerful close combat fighter of his time is a big threat anyway
he would be pretty strong but i have a doubt that he might not be able to use 8th gate for i think that you can only use it if you are alive
but of cause i don't know anything about that so i might be wrong
If they Edo him, then Naruto has hit an all new low. That is if they do, but in retrospect, if we are talking about just his power then of course he would be menacing.
I think the only weakness of edo tensei Guy would be the only time that his body has to renegerate after what pressumably will destroy his body, on the 8th gate being open or not?