[2200]

You hold:
A
Axx
AKQJxxx
xx

Partner opened 1H, you 2D， pd 2H(min but not necessary 6-bagger），after setting up hearts, you launched RKC, pd has “2 with”，is that all you need? Can't you count 13? Isn't it true that all you need is 3-2 break in trumps?

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So after 5NT K asking, GIB stopped at 6 level. What makes GIB so pessimistic？Knowing I would be the declarer?

Of course, this is GIB's normal level. Compare to numerous bizzare things, reaching 6H only is totally acceptable.

[smerriman]

Is this basic GIB or advanced? Basic can do no more than count points so could never bid grand with this few.

The ability to imagine what partner holds and "count tricks" is precisely what advanced GIB was built for. I would be extremely surprised if it did not bid 7.