A doubled nun carries shaddah and requires ghunnah, just as a doubled mim does. Nun has its own tongue articulation, but its doubled pronunciation also retains appropriate nasal resonance. Shaddah does not mean reciting two detached nuns.
«إِنَّ»
Indeed. The doubled nun carries ghunnah.
«جَنَّاتٍ»
Gardens. Its doubled nun is pronounced with ghunnah.
Compare: «إِنَّ» contains a doubled nun, whereas «مِنْ» contains an unvowelled nun. The doubled form has its own required nasal treatment; the still form is governed by the next letter. It is incorrect to give every occurrence of nun the identical treatment.