For a long time I never did this. If I was at the gym I'd have water. If I went for a run outside I didn't bring anything with me. Then I started bringing gels with me to help fuel my runs. Good thing? Yes and no.
If you're having gels during your runs with no water then you're dehydrating yourself even more because:
1. You're already sweating and losing water that way BUT...
2. The gels you're consuming with no water means that water from elsewhere in your body needs to go to your stomach to help dilute and digest the gel. This will speed up the dehydration process. Not something to be striving for!
Tips for fueling your endurance activity:
1. Bring water.
2. Drink it!
3. Use nutrition - whether gels/bars whatever you find you enjoy
4. Keep track of what you used and how you felt
5. Training (like life) is about trial and error. There's so much to learn if you want to progress in your athletic and/or life-related endeavours.
You will notice a huge different if you're fueling with water AND nutrition on your endurance-related training.