Because "increase" and "excrete" have completely different roots that only coincidentally coincide when the verbal nouns corresponding to those words are formed.
You mean, the difference between "going forward" and "coming together"? It's in the prefix, "pro-" (for, forward) versus "con-" (with, together) which give you different shades of the meaning. Can't really say what's the verb of movement was though.
I think he meant it as an absurdist joke, but this is a great response!
I looked it up, "gress" comes from "gradi" in Latin which directly translates to "walk". More specifically: con(pro) + gradi -> congredi (verb) -> congressus (noun)
Edit: Knowing this, "gradient" has an interesting flavour :)
Edit: It looks like the path is more indirect for "gradient"
"gradi" (walk) -> "gradus" (step) -> "grade" (french influence) + "salient" -> "gradient". I like that in Latin "walk" is "to step", or perhaps "step" is "the unit of walking"? "A walking"? Etymology is fun!