Definitely use CFW - getting Gateway working on a high enough firmware to be able to play any games will be fiddly, time-consuming and annoying to say the least.
First question is, do you want to keep the save games on any of the Gateway games? If not, the process is easy. Simply follow this:
https://3ds.hacks.guide/installing-boot9strap-(mset) (it uses the same exploit that you use to launch Gateway mode, so it's something you might already be used to and can access). If you want to save things like Mii Plaza progress, friends etc, also follow the optional "Move emuNAND" section when it is offered to you. Otherwise if you are happy to start with a blank slate you can ignore it. If you have any issues, or you want to keep the save games from your Gateway card, let us know.