The technical development of GPS watches is currently quite precise and accurate. However, problems can still occur if the GPS signal drops out, for example in a dense forest.
To ensure that the GPS determination is as accurate as possible, correction signals are sent from geostationary satellites. These usually belong to the European or Japanese system. GPS watches that do not receive these signals can have deviations of five to 15 metres. However, the other GPS watches are so accurate that they only record deviations of up to one metre.
However, your training data may still have errors. If you enter an area where your GPS watch suddenly loses signal for several seconds, your data may be corrupted because your watch does not know what you were doing during the time without signal.
If you spend more time with your GPS watch and observe your training, you will notice measurement errors and be able to eliminate them. In general, GPS watches are quite accurate and should have low error rates.