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I assume you mean if Guard Gomazoa is blocking a creature with trample, such as Garruk's Horde. If that's the case then all that is required is that Garruk's Horde (or whatever creature has trample) assign lethal damage to Guard Gomazoa, then the rest can go through to the defending player.

Just because Guard Gomazoa will prevent the combat damage done to it, doesn't mean that it will soak up all trample damage.

what he said + lethal damage means damage equal to the thoughness of the blocking creature (and not actually damage that would be lethal). Deathtouch is an exception to this rule. If lets say a 7/7 creature with deathtouch and trample would attack Guard Gomazoa it would deal 6 trample damage to the player and would only have to assign 1 damage to Guard Gomazoa even if that 1 damage is prevented.
1 Deathtouch damage is concidered lethal damage.
Trample also works on creatures with protection. if a 2/2 creature with protection from green blocks Garruk's Horde, Garruk's Horde still tramples over it.