"Gain knowledge" is a common collocation that even has its own dictionary entries. You might also use "acquire knowledge" if you wanted to write or speak in a more formal tone. "Get knowledge" wouldn't have a different meaning, but it's just not commonly heard.

"Gain" has a well defined meaning that can be found in a dictionary. "Get", on the other hand, means almost whatever the speaker would like it to mean. You can find "get" in the dictionary too, with hundreds of meanings. The meanings you find there are only the beginning, though. You can give "get" any meaning you like so long as it is loosely connected to some idea of acquisition or becoming. It serves as a rescue word, available for moments when you cannot think of a good word. So, if someone says "get knowledge" to you, you simply have to ask what they meant by that.

Hi! To gain or acquire is to grow in knowledge or wisdom. To get something is more instant or complete … I get a degree but I gain experience. Does that make sense?