If something gathers speed, momentum, or force, it gradually becomes faster or more powerful. Demands for his dismissal have gathered momentum in recent weeks The raft gathered speed as the current dragged it toward the falls. = gain
If you gather things, you collect them together so that you can use them. I suggest we gather enough firewood to last the night She stood up and started gathering her things together. = collect Gather up means the same as gather. When Sutcliffe had gathered up his papers, he went out He gathered the leaves up off the ground