Creatures with power less than Aura Gnarlid's power can't block it.
Aura Gnarlid gets +1/+1 for each Aura on the battlefield.
6/15/2010 The power of each potential blocking creature is checked against Aura Gnarlid’s power only as blockers are declared. Once a creature blocks Aura Gnarlid, increasing the Gnarlid’s power or decreasing the blocking creature’s power won’t undo that block.
6/15/2010 Aura Gnarlid’s second ability counts each Aura regardless of who controls it.