My name is***** and I think I can help you with your Gracie.
First I will explain normal dog toileting behavior and I think that will help you understand why Gracie is acting this way.
In a pack, dogs like to hang out close to each other. Then, when they need to toilet, they leave the group, find a private place and do their business. Dogs instinctually leave the group and their sleeping areas so that they are not soiling their living area. It sounds like Gracie has normal dog behavior when it comes to toileting.
The problem, of course, is that we as humans, want her to toilet outside. When we "potty train" dogs, we are asking them to prefer to toilet outside, usually on grass, but sometimes, mulch, gravel or concrete. The dogs see this training as "my people want me to toilet on grass/mulch/etc and give me praise when I do, so I should toilet in this location." They don't differentiate between inside and outside. Gracie has picked your office as her preferred spot because she wasn't clear on where you wanted her to toilet. It sounds like she is a smart dog, though, and with a bit of work, you can show her where you would prefer that she toilet.
The first step is to prevent her from leaving the family group when you are home. I recommend either baby gates or keeping her on a leash directly next to the primary person in the house. This will prevent her from "sneaking off" to her private spot to potty. Then you have to remember to take her outside to the dog toileting area very frequently. Once out there, remove the leash and allow her to sniff and explore. The instant she squats, use verbal praise saying something like "Good Potty!" Then once she is done toileting, give her a small treat immediately after she leaves the squat position. This will clarify for her that you want her to potty outside. This may take a few weeks to a few months for her to get it. The next step is to prevent her from having access to the office when you are sleeping or not home. I recommend using a crate and training her to go in the crate when you leave the house or at bed time. You can give a specific command such as "Gracie, Crate!" and show the crate to her and maybe place a treat in the crate to encourage her to use it. You can also keep a special chew toy specifically for the crate only (such as a Kong Toy with peanut butter).
Crates are great for dogs. When the dogs are trained properly, they become the dog's bedroom, their private place for peace and quiet. You can think of it like a fox den. They like the closed in space. I recommend keeping the dog's crate in the primary owner's bedroom, as this comforts the dog and they know they are not alone and isolated from their pack.
Gracie will figure out how to potty outside, but it may take several months for her to be trustworthy alone in the house. Remember, though, dogs like their crates and spending a few hours in there every day is not a hardship for them, it is nap time.
Good luck to you and Gracie. I have great hopes that she will learn to potty outside.
I hope this answer helps you. Feel free to contact me if you have any further questions.
Kansas State University
College of Veterinary Medicine Class of 2001