7.20e is still ok for gateway when I get one isn't it?
You will lose access to the easier entry point, but you can regain it through downgrading.
With 4.5, you will be able to use the blue card to play DS games, and can access Gateway Mode (emuNand, using 3DS games on the flash card, playing games installed via CIAs) through System Settings > Profile > Nintendo DS Profile, which will then reboot into the Launcher.
With 9.0-9.2, you will have to use the browser exploit, which involves opening the browser and going to a specific webpage, which then reboots into the Launcher. (There is an Android app that hosts the webpage, I believe, so you can connect to your phone rather than relying on an outside server.)
If you update past 4.5 but not past 9.2, you can downgrade back to 4.x. However, if you're buying a legitimate Pokemon card, then your options to play it will be to either not downgrade (so it can be played in sysNand), or to use Classic Mode each time you want to play it (which means rebooting the system and re-entering the Launcher each time you want to switch between Gateway Mode and Classic Mode).
Another option you'd have is that you can also use the Launcher to dump your physical game, and put the resulting .3ds onto the Gateway card and run it that way. Since you'll have copied your own card, it would have its own unique private header, and you wouldn't have to worry about being banned.
I've glossed over some things, since I'm not sure how much reading you've already done, and so I'm not sure how much you understand about how the process works. I'd be happy to go into more detail on anything you like, though. (I've actually been thinking about trying to put together some sort of graphical guide to help explain things to people coming in for the first time, but I'm out of town visiting family, so I haven't really had free time for the most part.)