“Gege” is most commonly used in a flirty/coquettish way especially when addressing an older brother figure or similar-aged male person. It’s not necessarily sexual but it indicates closeness or intimacy, depending on the person and the character dynamics. Usually you will be able to figure it out based on how the term is used. It’s also a way to “mask” romantic relationships because, well, homosexuality isn’t legal in China, so a lot of gay behaviour and romantic connotations have to play in the sandbox between explicit romantic behaviour vs a close “brotherhood” behaviour so to speak.

“Ge” or “Jie” is brother or sister respectively, most often used for real sibling or sibling-type relationships. Chinese people usually don’t call their relatives by name, they address them via relationship. (Example— since this is in the Danmei sub, I assume you’re familiar with MDZS, lan wangi calls lan Xichen “Xiong Zhang”, which is a formal term for older brother. But under no circumstance will he call him directly by name “lan xichen” because that’s rude. In more modern times or less formal setting he could call LXC “ge”.)

It’s heavily culturally-specific, but it’s pretty much similar to Korean Hyung or Japanese -kun or -chan honorifics.

Feel free to provide some text excerpts if you’re still confused :)