I am planning to build an indoor gate for my dog. I want to keep him off my workplace because. Which is better, the free-standing or the wall-mounted one? 

Look at the wooden baby gates that have an adjustment for different size openings, and a lever to close and hold the gate against the door frame. Easy to use and easy to move out of the way.

Better in what sense? It depends primarily on the size and strength of the dog, as well as whether or not you want to be able to remove and replace the gate. Wall-mounted is generally stronger, as it doesn't need to rely on friction to keep it in place. Most portable designs don't permanently alter the door frame, which could be important if you rent. Either could be aesthetically pleasing, depending on your skills for both design and execution.

I'm toying with ideas for a gate to keep the pitbulls in the family room, and I'm definitely going to need something permanently mounted. For the designs I have in mind, I'd have to space the elements closer together if I had a chihuahua and make the whole thing taller if I had a Great Dane. Your mileage may vary.

If your workspace is an outside building you could make a perminantly fixed full door gate, that would not only keep the dog outside when you need to have the door open to work but woud serve as a security grille when locked down. As stated it all depends on your situation, information you'e not supplied.

Have any of you looked at the dog gates from stately english houses? 

Thanks for the suggestions. Just an update, I found a wall-mounted pet gate when I was searching online. The dimensions seem to fit my workplace.