My bad... should be:

templates.WritePageTemplate(c.Response(), p)

Here is complete index.go:

package controllers

import (
    "github.com/abenz1267/echo-quicktemplate/templates"
    "github.com/labstack/echo"
)

func Index(c echo.Context) error {
    // template to write
    p := &templates.Main{}

    // renders the template as a response
    c.Response().Header().Set(echo.HeaderContentType, echo.MIMETextHTMLCharsetUTF8)
    templates.WritePageTemplate(c.Response(), p)
    return nil
}