Good topic and good question, I have no idea why no man responds.
Yes, responding 3nt is too worse, its only correct bidding should be to cuebid 3
♠ to show
♦ fit. ok?
However the above is only a guess!!! Never be a real option.
Now let me tell you its issues on Gib CC.
Now you see that 3
♠ says " 2-5
♣,2-5
♦,4-5
♠,15-17hcp,18-tps,forcing.", that's to say rebid-3
♠ will only show
♠ count instead of first control in
♠. So we can be easy to get conclusion - there is a issue on Gib CC.
After 3
♥, opener rebid 4
♣ is cuebid to show
♣Ace and deny
♠Ace, so you only rebid 4
♦ to show
♦ fit, but very difficult to bid up to slam on such Gib CC.
So I think you have no way to probe for
♦ slam for time being on such worse Gib CC.