The GMU (Gateway Migration Utility) uses Java. If you are using a Windows machine to run the utility, you will notice the amount of Java memory increasing while the utility is running. You can observe this via task manager.
The utility may stop at the same point in memory usage every time with the following error:
Execution failed. Reason: Java heap space.
The machine may have a lot of memory but be limited on Java Memory
Check the max amount of Java memory allocated to the machine.