pimp template system is based on Go template system. There are no fundamental differences between them. The following piece of code behaves as you would expect:
{{-/* template.tmpl */-}}Git branches in {{pwd}}:{{-range GitBranches}}*{{.}}{{- end}}
Note the use of the GitBranches function. Refer to the template functions documentation to learn more about all the functions available.
You can use pimp --eval 'funcs | toPrettyJSON' to list all the functions available when rendering a template.