Call a method from a Go template

Just omit the parentheses and it should be fine. Example:

package main

import (
    "html/template"
    "log"
    "os"
)

type Person string

func (p Person) Label() string {
    return "This is " + string(p)
}

func main() {
    tmpl, err := template.New("").Parse(`{{.Label}}`)
    if err != nil {
        log.Fatalf("Parse: %v", err)
    }
    tmpl.Execute(os.Stdout, Person("Bob"))
}

According to the documentation, you can call any method which returns one value (of any type) or two values if the second one is of type error. In the later case, Execute will return that error if it is non-nil and stop the execution of the template.

You can even pass parameters to function like follows

type Person struct {
  Name string
}
func (p *Person) Label(param1 string) string {
  return "This is " + p.Name + " - " + param1
}

And then in the template write

{{with person}}
    {{ .Label "value1"}}
{{end}}

Assuming that the person in the template is a variable of type Person passed to Template.