Putrid Goblin does not have a -1/-1 counter on it.
You have had at least one Zombie enter the battlefield under your control this turn.
Steps
Activate Thermopod's second ability by sacrificing Putrid Goblin, adding ({R} magic symbol) .
When Putrid Goblin dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When Putrid Goblin enters the battlefield, Geralf's second ability triggers, putting a number of +1/+1 counters on it equal to the number of other Zombies that entered the battlefield under your cotnrol this turn.