Energy gels can cause stomach upset in some athletes. This is especially true if you take the gels without enough water to help you digest them (8).
However, most people can train their digestive systems to tolerate energy gels during exercise without any uncomfortable side effects (8).
If you're new to energy gels, it's recommended to take half of a gel with water during a training run and slowly increase the amount over several training days.
Remember that some gels will sit better with you than others, so it's a good idea to experiment with different types prior to race day.