I don't think Gambia(nor Spartan) is the guy who defaulted.
Luffy is the 556th contestant, 556/4=139.
The numbers given for the blocks were B-138 contestants, C-139 and D-138, these numbers were stated in the chapter after maynard's debut. Later when the B block starts gatz still says it is a 138-man battle royale
http://www.batoto.net/reaead/_/167382/...by_mangarule/2
meaning maynard was just the 139th fighter of B block. Oda wasn't including capman when he told us there were 138 fighters in block B
That means the 139th fighter of D block was either gambia or spartan and Oda already took him out of the math.
Spartan was from block A and gambia was the 139th fighter of block D.
So block D has 138 fighters, neither gambia or spartan are between those 138. Gatz says someone defaulted, block D has now 137 fighters. The only known character who hasn't appeared in the ring is Meadows.
Donc ce n'est pas Meadows 