Not sure, but I think OP was just interested in building a route from A to B but taking the long way. ie. dictate the roads used. This is a route and it can be saved in .gpx format (others as well) when you use software like Tyre to build it. The real question is....can you hook the GPS up to a computer and transfer a plan (route/track), that you created yourself, to the machine. Not very many of the newer, automotive GPS units have this capability. There are some, but not many; especially the relatively inexpensive ones you see at Costco. With most of them, you tell them what our destination is and they either calculate the quickest or shortest and that is it.
You can make a route virtually the same as a track if you put in enough waypoints when you are building it (within the limits of the number of waypoints allowed).
The value of"tracks" over "routes" is that tracks can allow you to take roads that do not show up on a lot of maps. However, in order for a track to be created, the route has to be run first and the GPS will keep "track" of where you went utilizing many points. You can then utilize that track (load it back into a GPS, share with others, etc) to take you back along the exact same route. This is valuable for off road riding since many of the small trails do not show up on maps used by most routing software. eg. Tyre utilizes Google maps and as good as they are, you will not find a lot of small trails on it.
At least this is the way I comprehend it. Discussing GPS and utilization can quickly become an "oil/tire" thread.
Here is an illustration of what I mean. I will use Google maps to illustratre: I want to go to Great Falls, MT from Calgary, AB
This is a route that most of the newer auto GPS will give you. It will only let you enter where you want to go and then it will give you a couple of options.
https://www.google.ca/maps/dir/Calgary,+AB/Great+Falls,+MT,+United+States/@49.2448177,-115.0056181,7z/data=!4m13!4m12!1m5!1m1!1s0x537170039f843fd5:0x266d3bb1b652b63a!2m2!1d-114.0708459!2d51.0486151!1m5!1m1!1s0x5342378d658cb83b:0xd6de56b18e5945a8!2m2!1d-111.2833449!2d47.4941836?hl=en
Now, if I want to go a route that I want to go, I create a route by entering in multiple waypoints and it will give me a route that I want to ride.
https://www.google.ca/maps/dir/Calgary,+AB/Longview,+AB/Pincher+Creek,+AB/Choteau,+MT,+United+States/Great+Falls,+MT,+United+States/@49.3943475,-116.053097,7z/data=!4m52!4m51!1m15!1m1!1s0x537170039f843fd5:0x266d3bb1b652b63a!2m2!1d-114.0708459!2d51.0486151!3m4!1m2!1d-114.58905!2d50.9326409!3s0x53711ad66469c00d:0x4c88c6b4fc866eb7!3m4!1m2!1d-114.4632708!2d50.8048435!3s0x537107a4557b9abb:0x14b69051c631adae!1m5!1m1!1s0x5371b6fb5386db01:0x58895d27f2349637!2m2!1d-114.2333875!2d50.5313416!1m10!1m1!1s0x536f0a654fcc4e27:0xb161e144ca4fb420!2m2!1d-113.9502548!2d49.4863458!3m4!1m2!1d-113.2708305!2d48.5568718!3s0x536894f72b0164e7:0xd3018ee89fb4d05f!1m10!1m1!1s0x53428194bf213c65:0xb2dd5ca9a3382b06!2m2!1d-112.1836301!2d47.8124531!3m4!1m2!1d-112.0481611!2d47.0652064!3s0x5343231103010979:0x3ee2daeb5b068d98!1m5!1m1!1s0x5342378d658cb83b:0xd6de56b18e5945a8!2m2!1d-111.2833449!2d47.4941836!3e0?hl=en
This is what we want to create and then load into the GPS unit (with Garmin, it is usually as a .gpx file)
Google maps is limited in the number of waypoints and manipulations that you can do, but other programs (eg. Tyre) are a lot more flexible and allow you to to create a route down to very minute detail.
Rod