The .gist-vim
config file name is not open for customization.
You could ask the plugin author to make it customizable.
Why can't you set the variable in your vimrc though, you may want to know. First, you need to learn about Vim's variable name namespaces, documented at :h internal-variables
.
Variables prefixed with s:
are variables local to a script. See :h script-variable
. Thus you cannot access those from your vimrc.