The guide number of the Gemini 200 is 54m at ISO 100 for full power, so at f/8, you would need the strobe to be 6.75 meters away. The formula for this is:
GN = distance x f-number
So, from that you can work out your distances for the lights. Bear in mind that modifiers, such as umbrellas or softboxes, will reduce the light by some amount. You'll have to check the documentation that comes with them in order to be sure of the amount as there is no rule of thumb for that. However, lets say though, that it costs you a stop to have a bounce umbrella and you still want to shoot f/8. Then the a full stop lost from f/8 is f/11 (in other words, shooting with the umbrella is like stopping down to f/11 in light loss) and you would have formula:
distance = 54/11 = 4.9m
At any rate, I suspect you're fine with Gemini 200 as I'd be surprised if you got them up to full power in a close studio setting. You'll probably also do what I did eventually and buy yourself an incident light meter to get more accurate measurements.