Two options:
1. Lucho thinks that Gumbau is generally a Barca's level material
2. or Lucho thinks that both Grimaldo and Gumbau aren't good enough for Barca, but Lucho thinks that we are well covered at full backs (Alves, Alba, Vidal, Mathieu, Adriano, xx some other options, Vermaelen maybe if needed), so he doesn't count on Grimaldo in any way.
On the other hand, with midfielders we have "only" Rakitic, Iniesta, Roberto, Rafinha + Busi and eventually Masch.
-- so, maybe Lucho thinks that we need an extra man/option in midfield, and he trusts the most in Gumbau currently
More or less, it is sometimes irrelevant who is actually a better player (Grimaldo or Gumbau), when we are well covered on some positions, or at least Lucho thinks that we are well covered on some positions.
Also, Lucho probably doesn't see Grimaldo as a central midfielder, and he doesn't rate him too high for a left back or a left attacker (Ney's position)
++ Also, I forgot Rafinha in the post above, where I was counting how many players we already have...
You also forgot Samper, who is also on tour with the first team.
Let's be real here for a moment, Gumbau is not Barca material. He's just there to fill in the gaps.