Error: Missing value after --simulation
You may see this error in your flood logs which indicates that we were unable to programatically determine which simulation to run in your test plan. 
This can occur if the primary .scala file uploaded for Gatling test does not have the following pattern:
/class (\w+) extends Simulation/
e.g.
class YourSimulationName extends Simulation { ...
It can also happen if multiple .scala files are uploaded, in which case you should adopt the advanced scenario demonstrated here which uploads a zip file containing dependent .scala files.