Hi Phillipe,
I skipped SME 5.5 and anything to to with it altogether, so I am unfamiliar with your version of the freeswan howto.
The Freeswan Howto I use is the one from Shad Lords. While the freeswan how-to does not explicitly mention dynamic IP addresses, you will find that it mentions that you can use either a dns (or hostname) or an IP address for the remote address which means, you can use dynamic IPs (if you update your IP with a dynamic DNS provider) with that contrib.
Shad's howto does not require you to enter the remote gateway address either.
Kelvin