NBC has a perfect right not to show gun ads. Asking the government to force them to show gun ads because they show other kinds of ads is just as much anti-Liberty as the government trying to regulate your carry.
If you don't like what NBC is doing, don't watch. Avoid the other channels that NBC owns. I don't like what they are doing and, after verifying for myself their policy, will stop watching NBC until they change that policy and actually air an ad. Furthermore, I will make a point to mention NBC's policy to my firearm customers and encourage that they take consumer action.
Also, if I had Comcast cable, I'd dump it.
However, I support unequivocally NBC's RIGHT to choose its advertisers because the only way to truly advocate Liberty for myself is to advocate it for others.