How to remove zsh alias?

You can remove an alias simply using:

$ unalias gcm

This other answer is correct but if you're adding the unalias gcm line to your .zshrc file, it must be done AFTER oh-my-zsh is sourced, or else it will be overwritten by the zsh defaults.

It would look something like the below in your .zshrc file:

source $ZSH/oh-my-zsh.sh

# must unalias all ZSH defaults here AFTER we source the above
unalias gcm
alias gcm="whatever you want"

For removing git aliases in zsh git plugin you can comment them out in ~/.oh-my-zsh/plugins/git/git.plugin.zsh which is default installation location for git plugin