I don’t know much about Igus robots nor about a bridge to ROS.
You should write a program that reads the GCode and transforms it into robot program (depending on the programming language used on the Igus robot). This is not best as this is a conversion. You could also modify Slic3r so that it does not output GCode but instead directly Igus compatible machine code.
MoveIt will allow you to simulate the robot and do the path planning. It is not MoveIt that does the connection to the robot or a possible conversion.