Example sentences
wǒ我gē哥ge哥jié结hūn婚le了。
My older brother got married.
gē哥ge哥zài在shàng上dà大xué学。
My older brother is in college.
About this word
哥 is the word for an older brother, and Chinese simply says it twice. Reduplication is how sibling words are built, so 姐姐, 弟弟 and 妹妹 follow the same shape, with the second syllable flattened into a neutral tone. Two things trip learners up. First, 哥哥 is warm and spoken, while 兄 is the same relationship in writing and in compounds like 兄弟. Second, 大哥 means the eldest brother, but 大哥哥 means "a big boy" and is what a child calls an older boy. Chinese also forces a choice of older or younger for cousins, where English just says cousin: 表哥 is an older male cousin through your mother or your father's sister, 堂哥 one through your father's brother.
The story behind the character
哥
哥 is two 可 stacked up. Xu Shen wrote 哥, 聲也, sound, and noted that in older writing this was the character for song. Song. Later the language grabbed 哥 for "elder brother," and a new character was built for song by adding 欠 (breath) to make 歌. So when you say 哥哥, you are saying the old word for singing, twice.