The short answer is: 99% of the time, no, your Garmin's registered distance will not match the distance of your certified course, regardless of the course distance.
Note: While there are several GPS watches on the market, for consistency and simplicity, I will refer to one of the major brands, Garmin.
Certified courses are measured by the shortest possible distance you can possibly take through the course. The "lines" the person makes to measure the course are called tangents. Realistically, most people cannot run or walk perfectly on the tangents used to measure the course because of the number of people on the road in a race and because of normal activities during a race (water stops, passing people, etc). This is the main reason why you're more likely to get a longer course reading on your Garmin than the measured course distance.
For example, you might walk or run against the bank of a curved road because of the number of people in a race, but when the course was measured, it was done with straight tangents, not taking the full distance of the curve. This can add up quickly on courses with lots of turns. Also, most athletes do not know the tangents that were used to measure the course. The USATF measurement ensures athletes cover at least the minimum distance of the race; the reality is you will not perfectly cover the lines used to measure the course.
Your GPS will pick up any extra distance you cover from crossing from one side of the road to the other, crossing over for a water stop, covering extra steps to pass people, or leaving the course for any reason, such as going to the bathroom. It might not be much, but in some races where you have to really step away from the course to get to the porta potties, this can add a tenth of a mile. The more you weave in a race, the more steps you take, and the longer your GPS reading will be.
|The line is the measured course distance. How many of us follow the curves instead?|
Another common issue that affects what your Garmin measures is satellite interference. The Garmin is dependent upon satellite reception. There are known cases of cloudy weather, tall buildings, power lines and tunnels interrupting the satellite reception, and thus impacting the total distance counted by your Garmin.
The time you start and stop your Garmin also has an impact on the measured distance. If you're starting your Garmin at the back of the race pack or in your corral instead of at the start line, it can add an extra 10th of a mile easily, especially in large races. How many people always perfectly start their GPSes at the start line and stop them right at the finish?
I do believe that if your race is on a course that is not USATF certified, and the Garmins of you and your friends show a measured distance that is drastically different from the claimed race measurement, then you do have a reason to point this out to the race director. However, in USATF certified courses, wrong measurements of a course are extremely rare. There are rare cases of certified courses having to move slightly to accommodate unexpected issues, such as fires on streets the course would normally be on. In those situations, there's not much you can do.