Assuming I read your post correctly, you have two modules, each of them 8GB in density. Is this correct? If so, then the rest is valid, otherwise ignore it.
First off, what are the specs of each 8GB module, secondly what are the first 8 digits of the GSkill S/N and what is the ver# of the Corsair module? This info will be located on the label of each of the modules. The GSkill will start off with something like 1250 (which stands for the production date of year 2012 and the week 50), followed by the 4 numbers that indicate the brand of ICs under the hood. The Corsair will have something like "ver3.11" or "ver5.23", etc. This info is also an indicator of the brand of ICs used by Corsair on your module.
Depending on the chips used you might very well be able to run them together, especially at stock speeds.
Alternatively, return the GSkill and buy the single stick that this guy has for sale. He got it the same way you did, as a combo with a motherboard, so check with him and if yours and his match in specs AND version #, then going with a matched set of sticks would be the best route.
http://www.overclock.net/t/1362555/corsair-vengeance-8gb-cmz8gx3m1a1600c10-ram-module/0_40Edited by Reefa_Madness - 2/20/13 at 4:36pm