This is my current gas miner orbital station:
Hosho class Gas Miner 742,547 tons 2,510 Crew 10,760.5 BP TCS 14,851 TH 0 EM 0
1 km/s No Armour Shields 0-0 HTK 1333 Sensors 0/0/0/0 DCR 1 PPV 0
MSP 9 Max Repair 2400 MSP
Kaigun-Ch?sa Control Rating 1 BRG
Intended Deployment Time: 3 months
Fuel Harvester: 250 modules producing 10,000,000 litres per annum
Refuelling Hub - Capable of refuelling multiple ships simultaneously
Fuel Capacity 12,000,000 Litres Range N/A
This design is classed as a Commercial Vessel for maintenance purposes
This design is classed as a Space Station for construction purposes
It is flagged as a tanker with minimum fuel=0.
As Pedroig said, unloading directly to a colony requires a refuelling system. There is an easy work-around: Just have a tanker transfer the fuel before towing the station to the mining site. As you need the tanker anyway, this is essentially free.