What happen in this scenario:

- I have Grandmaster on the field.
- I activate Reasoning.
- I pick a Grandmaster.

?

grandmaster goes to the graveyard and you get no monster

unless the opponent calls it wrong he stays but if he calls it right then he dies typical reasong ruling

this exact scenario happened at SJC San Mateo round 7.

GaneFPC, you obviously don't know Grandmasters text if you say that.

I have Grandmaster of the Six Samurai on the field
I play Reasoning
My Opponent Calls 4 Stars
I pick up Cards and Hit a Grandmaster

Normally you would summon the Grandmaster at this time, but because you already have one on the field, the one that is picked by Reasoning will die and you do not get to look for another monster, because there can only be ONE Grandmaster of the Six Samurai on your side of field at a time. Simple as that.

excuse me my apologies, i didnt realize there was a GM on the on field already, shoulda read more carefully :P