Sure. With a bit of planning it shouldn't be difficult.
Now I'm assuming that you don't have a switch in your house that control these two remote lights, But, I'm I'm also assuming that the power for your workshop and gate light come back and are wired through the many breaker panel for your house. If this is true, Universale Powerline Bus (UPB) switches will likely work for you. It can travel long distances over the powerline.
So you'll need to replace the lightswitch at the garage/workshop with an UPB light switch. At the gate, what you will use depends on if you have a light switch there or not. If so, just replace that switch with a UPB switch as well. If there is no switch, you can buy an in-line UPB switch and wire it into the light.
On the HAI OmnPro II side you will need a UPB transceiver, a free serial port, and relatively up-to-date firmware.
Now, what type of lights are outside your garage and gate? The switches can control most any kind, but if its a mercury or low-pressure sodium light you might want to add in some noise filters. Incandescent, compact fluorescent, and LED bulbs are not usually a problem.
Without filters, it all should cost you under maybe $200 with some minor wiring total, depending on the type of switches you buy.