This achievement is for collecting 20 million Gald over the course of all your play-throughs
This will likely take a lengthy amount of time. Be sure to activate Double Gald option during New Game+ which will give you 2x Gald at the end of each battle. A couple of good ways to build this up is to:
1) Sell all equipment you do not need
2) Repeatedly complete the final Class 4 Administrative Zone
3) Sell your Salvaged Items(Expedition on the 20th Island can bring back some expensive items)
4) Sell everything before you start a new game+