Design wise? Gourgeist's features (its actual face, I mean) need to be more defined. In fan-art it looks good but in-game it just looks like a blob.
Stat-wise, Trevenant is really, really bad. Good attack, middling-to-low defences and a terrible speed. I have no idea what they were doing. It's a giant tree, it needs much better health and defense.
Gourgeist is let-down by not having Flash-Fire (its abilities are useless) and being a physical attacker with a limited physical pool.
