Flashforge Dreamer not able to read Cura created G-code file
For the Flashforge Creator model an alternative firmware is available; Sailfish. However, Sailfish is not available for the Flashforge Dreamer. As a matter of fact, no alternative firmware is available for the Dreamer as a consequence of the Flashforge Dreamer being closed hardware and closed source. 

Not knowing the flavor af firmware (as it is closed source and could well be using a custom firmware build) using Cura to slice models for G-code files gives no guarantee for successful printing.

To be able to use Cura you would have to switch the current board from the Flashforge Dreamer and replace it with a board that accepts Marlin Firmware. Note that there is no default configuration for this pronter available so this requires a little work from you to go through the Marlin configuration file and adjust various constants to make your printer work. [edit: As pointed out below in the comments, in the process you will lose some of the maximum speed the Dreamer is capable of!]

Another option is to buy a license of e.g. Simplify3D (note that I have no affiliation with Simplfy3D!) as they claim to support this printer. There could be other software suites that support your printer.