Note: This is not to be confused with gradualism in biology, which relates to speciation.
In geology, gradualism is the idea that geological features we see today are all the product of very slow, gradual forces. This is an outdated view. The opposing view at the time was catastrophism: that what we see today is the product of a series of catastrophes.
Gradualism used to be called 'uniformitarianism' but nowadays uniformitarianism is what we might call 'actualism'. Creationists usually refer to uniformitarianism as if it still meant gradualism.