I assume that by "scratches" you meant that the gasket surface had some mechanical marks but wasn't cracked (which would necessitate a whole different set of criteria for evaluation). You need to evaluate several things in this case: depth of scratches, orientation (radial or circumferential), length, type of gasket being used, and finally operating conditions (temperature & pressure) and service.
If the scratches are radial, going from the bore of the flange to the OD of the gasket surface, you may have a problem, depending on the length and depth of the scratches. If they are short and do not completely cross the gasket surface you might not need to do anything. If they are long they may require machining to prevent a leak. If they are circumferential and don't cross much of the gasket surface in a radial direction you might not have to do anything with them.
If it is more of a gouge with displaced material at the edges than just a scratch, manual dressing with a flat file will remove the "humped up" metal. If they are deep and you have concerns that the gasket won't "flow" into the void when bolted up you are probably looking at a machining job.
If the service is such that any hint of a leak (even low ppm level leaks) is unacceptable, a machining job is required.
If you have to do some machining to restore the gasket surface all is not lost as there are several companies around that can do machining in the field, including Furmanite, Koppl, and Team Industrial Services.
Bottom line: gasket surface irregularities don't automatically mean a field machining job is in order to obtain a good seal.
the contact surface roughness for tongue and groove cannot exceed 125 microinches.