[levier17]

why is it that after openning 1NT - 2M - P - P
2NT is invite to 3NT? that doesn't make sense to me.

[Stephen Tu]

Probably because undefined 2nts are falling back to some default invitational to 3nt rule. GIB probably needs to be taught scrambling and minor-oriented 2nts in balancing / takeout double over opp's 2M situation.

Note that good player is going to bid 2nt here with frequency pretty much never. One might double with some 2(443) for takeout occasionally with no spade wastage. If you decided to open some offshape 23(26) 1nt I suppose you might sometimes try 3m, more often at MP. But most of the time you are supposed to pass. Especially against GIB who makes a lot of idiotic 2M Capp on 4x(x5+) hands, they are often in a terrible contract and you are rescuing them.

[pigpenz]

I think you will find the same problem the other way
1NT pass pass 2bid
pass pass 2NT

at matchpoints alot of players will play for +120 or -50 versus the other
score of not setting the opps enough or letting them make on hands where they have 7HCP
GIB just doesnt handle these well.