I tend to go with the SIS stuff so not sure differences. I'll try to blag this. If something is Isotonic which I presume High5 isogel is. You shouldnt need to drink water with it as the solution should be right for your stomach to absorb as it is. If however its a thick gel you will need to drink water to make it Isotonic so your body can absorb. As to how much I wouldnt like to say but you should be drinking enough for it not to matter too much.
Firstly mak sure you get some training runs in whilst using them. Dont just do it on the day.
Secondly do you absolutely need to use them? The reason for gels is to replace the carbs running uses. You can run up to 90minutes roughly before needing to reload. So if you are running a 1.40 - 1.50 ish HM you may not need to bother.
Also another thing to think about is when you are running at a nice gentle pace say during and ultra your stomach can absorb things better because it gets a normal blood flow. However during the quick pace of a HM your body pumps blood to where it needs it at the time and not to your stomach. So digesting food its alot harder and takes a lot longer.
Thats my 2 cents. Their are much more wise heads out there than me and may say different but i'd stick with Isotonic drinks for the duration?