Someone with Bisping's grappling has nothing to fear against a fighter like A. Silva who never really tries to take people down with grappling (his takedowns are actually knockdowns). Bisping won't be able to let his hands go against someone with Weidman's wrestling. Gastelum is a stocky guy who has excellent grappling of his own, so he's hard to takedown and Weidman took him down in the first round easily (they were still dry) and still with relative ease later on.
Bisping is still a tough fight as he's one of the most well-rounded fighters in the UFC, and he always comes in shape, but Weidman overwhelms him in grappling so much that it will turn Bisping into a striking amateur (See Sonnen/Silva 1&2). Sonnen's striking is not and will never be anywhere near Silva's, but the threat of the takedown allowed Sonnen, of all people, Sonnen, to catch Silva multiple times with punches.