Until martini uses cgo or unsafeand syscallpackage this should be fine .
README of martini contains an example of using martinis with GAE, as @elithar pointed out :
package hello
import (
  "net/http"
  "github.com/go-martini/martini"
)
func init() {
  m := martini.Classic()
  m.Get("/", func() string {
    return "Hello world!"
  })
  http.Handle("/", m)
}
.