I've heard that gel packs can help performance on long runs. How should I use them? How long of a run should I use them on, and how often during the run? Should I take them with water at a water station?
I'm not sure there are any definite knowledge on the use of gels on long runs. Which is reflected in the many different forms of energi products on the market - pastes, gels, gums, bars, etc. And the many discussions on the various running sites...
When it comes to gels, have a look at When to eat Energy Gels in the Marathon - which is fine, if a little thin. The links at the end are pretty good. As you're from US, this comparison might be of interest as well. (We have other products in Europe, but I have never found a similar page for these...)
Personally, I use a isometic gels from Maxim for every 7 km on longer runs. Which then means 2 gels for a HM (at 7 and 14 km) and 5 for a M (at 7, 14, 21, 28, and 35 km) assuming there is some sort of refreshment at the end of the race or training pass. This might be in the low end compared with what many others recommend - typically one gel per 20 minutes.
Other links, that might be of interest:
The underlying issue is that for a race longer than about 30km, you have to eat something, because your body cannot store enough energy to finish the race. That's the "hitting the wall" thing, and it's why a marathon is significantly more difficult than just twice a half marathon.
So for a half marathon or shorter, you shouldn't really have to eat anything on the way. (Drink yes, but not eat.)
A secondary point is that by the time the food has taken "effect", the race might already be finished.
The consideration is different during training, if you haven't had the opportunity to properly eat before a long run, as you would before a race.
Now the gels are just a way to absorb a lot of energy in a short time in a way that's convenient for a runner, since you aren't going to be cooking pasta on the way. There are other options as well, such as bars, bread, or bananas. You need to find something that works for you. Not everyone tolerates all the different options in the same way.
In a professionally organized race, they will have food stations at regular intervals. That should give you a hint at how many times you should eat. Three to five times is probably good. You need to find out what works for you.
And yes, the gels I'm familiar with explicitly say to take them with water. I usually take one just before a water station.