"Gan Lin Nia" is a really impolite slang in Taiwanese. The meaning of "Gan Lin Nia" is "f**k your mother". I would suggest you not to say this phrase in any circumstance. However, you probably want to understand the meaning of this slang when someone say it to you.
Example:
Someone: "Gan Lin Nia"!
You: "Gan Lin Nia", too!
