Here, knowledge is compared to a river. The quote says that some people "drink deeply" (take large mouthfuls to drink) from this river. It means that they take a lot of knowledge to their life. When someone "gargles" a mouthful water, they move it around their mouth and spit it out without swallowing. Some people "gargle" the water from the river of knowledge as well. They receive the knowledge but they push it back out, without taking it into their life.

It's a metaphor, comparing knowledge to water. "Drinking deeply" means that they are enthusiastic about acquiring lots of knowledge. Gargling refers to using a liquid to clean your mouth and throat by blowing air through it, and then spitting it out. People who "gargle" from the river of knowledge only expose themselves superficially, and so they fail to assimilate the knowledge.

“Drinking from the river of knowledge” is a poetic way of describing the process of learning about the world. “Gargling” is what you do when you put something in your mouth in order to clean it and then spit it out. (We do that with mouthwash for example). Thus, this is a joke saying that while some people drink deeply from the river of knowledge (they study hard to learn lots of things) others only take a little water from the river, gargle it around a bit, and then spit it out (they don’t learn much at all).

haha, this is a funny quote by Woody Allen. It's very metaphorical / poetic. 'To drink deeply from the river of knowledge' means to take advantage of learning opportunities all around you - and to "nourish" and benefit yourself with that knowledge. So this is funny when Woody says that 'Some people only gargle,' - because instead of nourishing themselves with knowledge or "ingesting" the knowledge - instead they are just gargling it in their mouth and spitting it back into the sink, and never really benefiting from the knowledge that is right there in front of them which is freely available and accessible. I hope that my explanation helps you understand it. :)

They way I understand it "to drink deeply from the river of knowledge" would be to swallow, intake or learn the knowledge. But to "only gargle" means you spit it out only taste but not accept the knowledge.