See the results of this poll: Which spelling?
Respondents: 28 (This poll is closed)
- Gianna : 21 (75%)
- Giana : 5 (18%)
- Johnna : 1 (4%)
- Johna : 1 (4%)
- Jonna: 0 (0%)
I’d stick with [name_f]Gianna[/name_f]. The others almost seem too forced, and a quick correction is all it takes? I love the Italian pronunciation of [name_f]Giovanna[/name_f], but I’d never opt for [name_f]Jovana[/name_f] to get to it? It also looks pretty, but takes away from the Italian charm in my opinion. 
Good luck!
I don’t see how [name_f]Johnna[/name_f] and [name_f]Gianna[/name_f] would have the same vowel sound. [name_f]Gianna[/name_f] had an open ‘a’ sound that does not exactly rhyme with o. I would stick with the [name_f]Gianna[/name_f] spelling.