The "gunk" is basically dust and oxidisation built up over the years, natural "rusting" really, that forms a dirt barrier to prevent the game cartridge working as it used to do.
This is a very basic tutorial that costs nothing to fix the problem.
For the purpose of this demonstration, I have used a Nintendo SNES game cartridge however of course this tutorial will work for any other game cartridge as the principles are exactly the same.
Step 1: Open Up Game Cartridge
Easy and cheap method is to get an old screwdriver and file the end into a groove so that it just fits over the recesses in the screw and will therefore turn it.
Then remove the screws.
Step 2: Clean the Board
You can clearly see that half of the traces have been cleaned and the other half are uncleaned to see the comparison. It just takes a few strokes of the pencil eraser, quick and easy!
When done one side of the board, turn it over and do the other side of the board too.
Step 3: Re-assemble
The traces on the game cartridge will now work properly and your game should now work!