Gray Merchant dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Gray Merchant enters the battlefield, causing each opponent to lose at least five life and you to gain that much life.
Activate Hex Parasite by paying ({1} magic symbol) and two life, removing the +1/+1 counter from Gray Merchant and giving Hex Parasite +1/+0 until end of turn.
Repeat.
Results
Infinite colorless mana.
Infinite death triggers.
Infinite ETB.
Infinite lifegain.
Infinite lifegain triggers.
Infinite lifeloss.
Infinite LTB.
Infinitely powerful Hex Parasite until end of turn.