Counter Bb2!
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Well black could try Nc6 and bd6 to fortify e5. I think sneaking in d5 is overly ambitous and black should be content with one pawn in the center for the time being. Oddly enough, the mainline here ends up with both queenside knights on the rim with: 1 b3 e5 2 Bb2 Nc6 3 e3 Nf6 4 Bb5 Bd6 5 Na3 Na5 6 Be2. White is now threatening to play Nb5, but black is able to hold the balance and maintain equality with the prophylaxis of 6...a6! That being said, the best option in this position is to get rid of the kings 7 protecting pieces and then kill the white king with love.
1.b3 e5 2.Bb2 d6 3.e3 g6 4.Nf3 Bg7 5.d4 Nf6 is a solid defense. The idea is that any dxe5 will be met with ...Ng4 taking advantage of the unprotected Bb2. White's e pawn is now pinned and Black can wait to capture back in the best possible way.
it is worth noting that in GCB's line, Black should be prepared to give up castling rights after 6 dxe5 Ng4 7 h3 Nxe5 8 Nxe5 Bxe5 9 Bxe5 dxe5 10 Qxd8 Kxd8
@Iberian_Wolf thanks for the insight! nice thought on d5. In 1 of the games I played d5 and then Nd7 blocking my c8 bishop, then ended up struggling 
I will follow this advice.
What is the rationale behind knights on the rim in the main line? For White may be not to block the Bb2? But for Black?
Just hold the ...e5 point. Sometimes you can even blunt the bishop with ...Nd4. White is developing a darksquare strategy so maybe you'd like to get going on the light squares?
@Deathbyblitz Na5 prevents the knight from coming to c4 as well displacing the bishop on b5. The is that once the a3 knight is restricted, black makes the argument that his knight can return to the action faster than white's
Often, you read about advantages of fianchettoed Bishop. But not much analysis on how to counter it. What is Black's strategy to blunt the deadly potential of Bb2? How does one plan opening moves given cramped setup of Black's pieces?
Any help/insight is appreciated. Better yet, post some games with analysis