B737 Captain reported while holding short of the gate they didn't have sufficient foot pressure on the brakes resulting in the aircraft rolling backwards until the Crew applied brake pressure.

Narrative

While holding short of gate XX waiting for marshallers; the aircraft rolled back approximately 5-10 feet due to insufficient foot pressure on the brake pedals complicated by a relatively steep undulation in the ramp. I thought I was pressing the brake pedals hard enough but it did not stop the aircraft from rolling backwards several feet. The FO and I both noticed this so I immediately applied stronger brake pressure which stopped the aircraft. I was surprised at the effect this hump had on the aircraft as it wanted to roll back even after I pushed the thrust levers up to taxi forward to the gate. I held the brakes until the engines spooled up enough to overcome the effect of the hump. We taxied forward safely to the gate without any other issues.