How to get underlying value from a reflect.Value in golang?

A good example of how to parse values is the fmt package. See this code.

Using the mentioned code to match your problem would look like this:

switch val.Kind() {
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
    m[typeField.Name] = strconv.FormatInt(val.Int(), 10)
case reflect.String:
    m[typeField.Name] = val.String()    
// etc...
}

Basically you need to check for all available Kinds.