Well, could you walk me through it in detail?
You already have gateway right, this means you most likely already have emunand and unlinked nands.
Put rxTools.dat and firmware.bin into the root of your SD card, you can also put the 7.X key if you want to decrypt 7.X games (optional), then you can use Mset to launch rxTools by installing rxtoolsinstaller.ds with a flashcard if you are on 4.2 or go to launch.rxtools.net and will load rxTools up to 9.2. Is as easy as thay to "install rxTools", is just like launching gateway's launcher.dat doesn't need to install anything, just launch it.
To install games you need a CIA installer, you can use FBI or Big Blue Menu, to install BBM you need FBI, there are 2 ways I know to get FBI, 1) Inject it on your NAND or 2) You can use Cubic Ninja + Pasta CFW to use FBI without installing it on your NAND, I have a Sky3ds so I just used a cubic ninja rom in my flashcard instead of buying a copy of cubic ninja, don't know if you can do that with Gateway card.
There are noob friendly YouTube videos on how to do either of those and myself being a total noob not knowing what I was doing was able to follow tutorials without issue.
Once you have FBI, you can install Big Blue Menu so you can acces it from the main screen.
It sounds really complicated when you don't know it but it isn't really that complicated once you do it.
Can you link me to the MSET exploit on 4.x?
i know how to downgrade
On the Gateway's files that you downloaded, there is a folder called bluecard, inside there is a file named gatewayinstaller.nds just put this inside a regular R4 card and run the file from your R4 like a normal game, follow a few prompts and it will automatically install the DS profile exploit for you.