I'm not sure it this is a problem with GIB or I'm confused about help suit slam tries. I've run into this when I was trying to reach slam, but in this deal we're looking at two GIBs bidding with each other.
My link
My understanding is that the 3♠ bid by West says "tell me (by bidding 4♠) if you can fill in the gaps in my spade suit, because that would give us a shot at slam," but rather than bid 4♠, holding ♠AJxx in the suit East responds 4♥, which appears to be a sign-off.
Page 1 of 1
Response to help suit slam try
Page 1 of 1