Regardless of what the base metal is, when electroplating gold the first part of the process is to lay down a layer of copper, which acts as a firm surface for the gold to 'bond' into. So, you may have just discovered the thin copper layer under the gold.
Electrolysis will do a good job of removing the gold, but will also remove the pretty copper under-layer too.
Now you may be lucky, and have a necklace that is, at base, made of a copper alloy, but equally it may be made of a ferrous alloy or the dreaded 'pot metal'. It would probably be worthwhile to:
a. see if it's attracted to a magnet (it's ferrous
b. find an area that won't be noticed and file away until you're sure you're through all of the plating. If it looks silvery, you're out of luck.
Of course, you could always re-plate it with copper, regardless of what the base metal may be.