B737 Captain reported they were unable to taxi the aircraft completely to the gate due a grate on the ground and a concrete slope and rolled backwards uncommanded. Aircraft was tugged into gate.

Narrative

Taxiing in to gate XX; we were making a left turn from the west with the #2 engine shut down. We slowed to await ground personnel to get into position. However; once we attempted to power up to the gate the aircraft would not move. After determining one engine wasn't sufficient; we started the number #2 engine. Again; we attempted to taxi up to the gate; but after applying 45% RPM on both engines; there was no movement. When I pulled the power to idle; the aircraft began to roll backwards. So; I set the brake and called operations. I asked them to have the ground personnel to inspect the tires and surrounding area for damage or reasons that the aircraft might not move. They initially didn't find and problems and hooked up a tug to pull me in to the gate. However; they did NOT plug in a headset or communicate with me. So; we gave them hand signals to stop and called ops to tell them they needed a headset to communicate with me. Once they did that; they backed the aircraft up to get a little momentum and then pulled me over the grate and parked us at the gate. After I spoke to the ground personnel; they explained the grate situation and the concrete slope around it that was preventing me to move.