You still need to pay the cost to cast Sproutback Trudge from your graveyard.

Sproutback Trudge’s ability counts the total amount of life gained without considering any life you lost during that turn. For example, if you lost 3 life and gained 4 life earlier in the turn, Sproutback Trudge will cost {3}{G}{G}.

The cost reduction can reduce only generic mana in a spell’s total cost. It can’t reduce the cost to cast Sproutback Trudge beyond {G}{G}.

You need to gain life before the end step begins for the last ability to trigger.

To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Sproutback Trudge’s first ability). Sproutback Trudge’s mana value is always 9, no matter what the total cost to cast the spell was.