If you're able-handed, you can sometimes unstick yourself without dismounting by engaging the emergency all-wheel-drive option ie. propelling the front wheels by hand inna wheelchair stylee. I've done it to good effect on snow, and I expect it's similarly effective on light gravel. Of course, you probably don't want to do that if the slippery surface is mud, especially if it's likely to contain poo.
Carrying plenty of momentum into the slippery bit works well for things like wet canal bricks, but Silly Sustrans Gates™ preclude that approach.
