There is a plugin that adds support for Makerbot (except for 5th gen) and FlashForge printers: OctoPrint-GPX by @markwal.
Makerbots up to Gen5 and clones talk s3g/x3g instead of Gcode. That's why you print the x3g file instead of the gcode file. GPX is a tool that maps from gcode to x3g. The OctoPrint-GPX plugin wraps GPX for two-way printer communication.
See the instructions on OctoPrint-GPX.