To mate Gargraves switches to Lionel tubular track, you will need to use the Gargraves mating pin #801, as shown in the FAQ section of the Gargraves website.
Mating the GG switches to Lionel track will be easy. With adapter pins, and in some cases shims to compensate for height differences, it's easy to mix brands of track and switches. You're going to find that not all trains and switches are compatible. I contacted Gargraves 2 years ago when I started designing my track plan, and they told me their switches wouldn't work for the trains I'm running (prewar tinplate for the most part). While I was disappointed, I also appreciated their honesty as I was prepared to spend a significant amount of money on switches.
With that said, the most important thing to consider when shopping for switches is what trains you are running. If you're running post-war or modern trains, you'll have more options. If you're running older prewar tinplate, you will have fewer options but there are still options out there that will work.