You have to keep your weight on the middle seat. You also have to keep a nice amount of heeling, that also helps to have the sail set quite well. Keep the tip of the boom well overboard and she is "speedy" with almost no wind at all. Under these superlight circumstances she still steers very well, you will have sufficient "feeling" about her balance and tuning. In this perspective it is one of the nicest boats I have ever sailed in these very light-wind conditions.
When the wind completely died I have rowed her a long way back. I have kept the rudder down and fixed the helm with a rope in the middle. I lifted the centerboard by about 30-40 cm and than she rows and tracks very well at reasonable speed.
I took my outboard with me for the first time. It's an older 2Hp 2stroke, weight 10kg. Plenty of power to have a GIS speed up very well. I used my rudder to steer and kept the centerboard down. It is nice that there is plenty of stability if you need to sit or kneel on the aft deck to handle the outboard. But keep your weight on the middle seat to maximize speed. It is nice to see that the hull hardly makes any waves and runs so smooth.
I think it's quite useless to buy and take an outboard on GIS in case there is no wind; GIS rows very well and you will get her anywhere rowing.
But an outboard gives more overall speed and could be helpfull if you need to go against waves and wind, when there is no space to tack or in busy harbours. I already had a small outboard, but I probably would not have spend money to buy one for my GIS.