It's definitely a hard fight to cap. Let's be honest, T-Wood was a total bore in Strikeforce and could have lost to Coy, Saffedine, Mein, and Daley. He has a good wrestling pedigree, but he does nothing when the fight hits the mat. He's made a name for himself in the UFC by landing some early KOs.
Conversely, Gastelum has less of a grappling pedigree, but he's pretty damn good once the fight is actually on the ground. He's tougher and more of a grinder than Woodley. He hasn't shown huge KO power like Woodley though.
I would definitely cap Woodley as the favorite, as he's faster, stronger, a better wrestler, a better striker, and has more KO power. Gastelum will have the advantage that he's in no danger on the ground, has better cardio, is tougher, and more active.