One key parameter for gas fuels to turbines is the Wobbe no, which in your case would vary considerably between gas obtained from a coal gasifer and natural gas.
The Wobbe number is expressed as HHV / (gas specific gravity^0.5). Expressed in US units ( Btu / std ft3), it can vary from 300 for medium btu gas to 1300 for natural gas. The range in fuel gas wobbe number acceptable to a GT should be specified by the GT manufacturer and is characteristic of the burners used in the combustion chamber.
In some cases, the acceptable range is further constrained if you have a DLE low NOx GT. Also, if you plan to make online changes to fuel gas source, the rate of change of wobbe number also may also apply.