Actually there will be a gap between your F2 & H1b. Since you'r a F2 now and not authorized to work in US, you will need to wait for your H1b approval which normally takes 3-6 months. Unless you choose premium process, which would normally take 14 bz days as well. Besides, don't forget about processing time to get all paperworks ready before you file. So, gap does exsits.