Gothic Riding Suit is crafted with Tailoring in Mabinogi (clothes). This page lists everything needed to make it, checked against the game as it exists today. The interactive planner lives in the Tailoring Companion.
How to get the sewing pattern
The sewing pattern is rank 2 and survives 30 uses; every crafting attempt spends one use.
Secret Experiment in the Underground Waterway Reward: Clear this Tara shadow mission from Sinead in the castle at any eligible difficulty.. Requires a Secret Experiment in the Underground Waterway Pass; not available in Basic or Elite difficulty.
Sources that no longer exist: Lorna & Pan's Gift Box (2011).
Materials
Expect about 8 attempts per finished item (13% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.