With the 20 amp gfi's in the kitchen can I provide a 2 gang box and feed the sw for an above the sink light from another circuit?
Yes you can as long as the circuit you are feeding from is not required to be a dedicated circuit.
As a side note GFCIâ€™s are not required in the kitchen as per the current CEC codebook (maybe Ontario is different?). I have read on this site I believe, that the next code session will require one within one meter of the sink.
Also a 20-amp circuit is not a requirement in the kitchen, as an alternative a three wire split circuit may be used.