If you use the Pro version, then the iframed content should be on the same domain and you will need to include the language code in the src of the iframe tag.
If you use the free version then you will need to have the free module enabled on the iframed page.
What do you mind with "include the language code in the src of the iframe tag" witch code? all gtranslate html code? and where exactly i have to add it? thank you regards Davide