Getting Roms from the extracted Romfs folder from GBA working in other emulators
Disclaimer: I support in no shape or form any way of Piracy.
Get the Game in a legal way.
I only post this as a form of a tutorial.

1. Homebrew your Nintendo Switch Console via the RCM or Modchip solution
2. Get a subscription to "Nintendo Switch Online + Expansion Pack"
3. Download the Emulator Pack you want to use (in my case the NSO GBA Emulator)
4. Start the application at least one time to generate the keys you need to dump with Lockpick_RCM
5. Dump the keys
6. Now there are two ways to extract the Romfs folder
6.1 You can dump the romfs folder directly with NxDumptool (will take a while since its a lot of files)
6.2 You dump the NSO GBA Emulator as .nsp files and extract the romfs via a Switch emulator (I suggest Ryujinx)

(I did the following Steps now on a Linux Distro)
7. Locate your Game inside the romfs/titles folder (e.g. Zelda Minish Cap would be inside the "A-8665_e" folder)
8. Inside there is a file with the ending .gba which is actually a archive and not a rom
8.1 I already tested if this .gba file will work on mGba. It does NOT.
9. Rename this archive to the ending .srl (.SRL clarification just in lowercase)
10. Now extract this archive
11. Inside the new extracted folder should be a file without any ending. Give it the Ending .gba
12. Load the file inside a GBA Emulator such as mGba
This was tested and worked on a Steam Deck inside the SteamOS Linux Distro.
As of now i dont know of a way to convert it to a working archive on windows.

Congratulations
You can now play the GBA Games

 

[Technicmaster0]

Took a quick look at it and found a solution for windows. The files seem to be lz4-compressed.
From step 8. onwards:
-rename the .gba file to .lz4
-Download https://github.com/lz4/lz4/releases (win32 or win64 zip file)
-Extract the lz4.exe to the same directory as the .lz4 file
-Drag the .lz4 file onto the .exe file
-you get a file without extension in the same directory. Rename it to something.gba

Also another note:
As long as you don't start the GBA NSO app, you can still dump the file system with nxdumptool. That means that you can cancel the NSO subscription, update the app and dump the current FS with nxdumptool as long as you don't start it again, even if you're not subscribed anymore. Of course this is not a recommendation to do that, only a small note that it would be possible theoretically

 

[Random_Emulator]

Took a quick look at it and found a solution for windows. The files seem to be lz4-compressed.
From step 8. onwards:
-rename the .gba file to .lz4
-Download (win32 or win64 zip file)
-Extract the lz4.exe to the same directory as the .lz4 file
-Drag the .lz4 file onto the .exe file
-you get a file without extension in the same directory. Rename it to something.gba

Also another note:
As long as you don't start the GBA NSO app, you can still dump the file system with nxdumptool. That means that you can cancel the NSO subscription, update the app and dump the current FS with nxdumptool as long as you don't start it again, even if you're not subscribed anymore. Of course this is not a recommendation to do that, only a small note that it would be possible theoretically

Awesome

Thank you for finding a solution for Windows User.

Appreciate it.

 

[Deleted member 413168]

Technicmaster0 is correct that they're lz4 compressed. I documented the exact flags they used to compress them (you can test this yourself you create exact matching compressed files). It's in the wiki for my gba emulator...as I didn't know where else to put it lol.
https://github.com/ITotalJustice/notorious_beeg/wiki/Sloop

 

[notsu]

6.2 You dump the NSO GBA Emulator as .nsp files and extract the romfs via a Switch emulator (I suggest Ryujinx)

I haven't tried it with NSO but doesn't DBI let you browse the romfs folder and send it over to another device directly?

 

[Technicmaster0]

I haven't tried it with NSO but doesn't DBI let you browse the romfs folder and send it over to another device directly?

Nx dump tool also has the ability to dump the file system directly