How many GUs for a HM?

1 or 2 max. It would likely be more for morale than anything. But whatever gets you there. If having a gel makes you happy go for it.

But like the others said; not necessary

I run a similar pace at that distance. One is gu probably enough, unless you felt really weak/tired on your previous ones. I have run plenty of HM and all my marathons without anything but water and sports drink; did take one gu (the kind with extra electrolytes and caffeine) for my last race as they serve nothing but water on the course, conditions can be very hot/humid, and I'd been having GI issues on and off all week, so it's possible I was starting a bit dehydrated. More for the electrolyte boost than the sugar. I believe I carried two but only ended up taking one (around 7 miles). Later in the race, I was running too hard and couldn't have gotten it down. Race nutrition is very individual, but you should be fine with nothing, or one gu, for a race that's "only" in the 90-minute range. I really think that most of what people think is "bonking" is actually just starting too fast for current fitness level and then paying the price later in the race. Gu won't prevent that! Take an extra along if you think you might want it, though, and play it by ear.

I finished my last HM in 1:37 and didn't take any GU's.  I was feeling a little weak the last 10-15 minutes so I'd say 1 at the beginning of the race and 1 at the 45 minute mark and call it good.

Going forward I'll be doing this.

Really depends on whether you hydrate with H2O or sports drink. For optimal performance best scientific evidence (Am College Sports Med analysis) suggests consuming ~30-60g carbs/hr for "intense exercise" >45-60min.  So for 1:30 HM the "best" answer ranges from  0 (min rec carb intake from all hydration with std carb sports drink) to 3 (max rec carbs assuming just H20 hydration).  Read product labels & use what works best for you. 

FWIW- I'm slower (1:50's HM) & don't like to rely on mix quality of event's sports drink.  I usu take 2-3 gels with plain H2O.

I took none in my last HM...1:41.  I brought one gel with me, but I was an idiot and accidently threw it away with an empty cup at the first aid station.  I was holding it in my hand because the shorts I was wearing that day didn't have pockets...LOL.

I was fine on just using sports drink the race provided.

From a physiological/nutritional standpoint, you should be better off with zero in a standalone HM. (Different story in a Mary or HIM/IM.)

Unless you start out with serious depletion in muscle glycogen, you really shouldn't be able to burn through your available stores in a HM, however fast or slow you are within the normal range. Your stores should be more than enough to get you through a HM, and at HM intensity most people will also be deriving some percentage of their energy from fat burning (lower percentage for faster runners, roughly). If you're struggling at the end of an HM, it's unlikely to be due to depleted fuel stores. It's due to other causes of fatigue. So if you've got enough fuel stored up to get you through the race, why take on external fuel? That would be useful only if digesting and metabolizing the external fuel was easier on your body than working with the stores that it already has. As far as I know, it's not easier. So better to go with no external fuel. You have plenty of time to refuel after the race. [Please correct this if I'm missing something in this reasoning.]

Things are different in a Mary or long-course tri, because those events are long enough to run through your supply of muscle glycogen. So in that case you need some external fuel, or you'll be reduced to fat burning alone (not so easy while exercising).

I did a 13 mile training run yesterday, my longest in many years. My legs were tired in the last couple of miles. That wasn't because I had used up all of my easily-accessible energy stores; it was because I'm not yet accustomed to 90+ minutes of pounding.

For me one gel at about 55 to 65 minutes into the race typical finish is between 1:40 to 1:50.  

I tried a second gel in one race taking them at about 40 and 80 minutes, for me it made my stomach hurt so I don't do it anymore.

I'm usually 1 or 1.5 for a 1:28ish. One at mile 8 and then half at mile 11. That .5 at mile 11 is mostly about security more than anything for me, if it's a PR attempt, I'm pushing pretty hard there and I just don't want to bonk with a mile or less to go. But I usually can't stomach more than a half for the second dose.

Same here! 

really? 4? at what point do you take them?

For me: One 15 minutes before and at the one hour mark.  I honestly doubt I need the calories but as a coffee-o-holic, I like the 35-50mg caffine boost from a gel at the one hour mark.  Caffine makes me happy, a happy me runs faster. 

This ^^^^^

If you're doing that pace- you'll be pretty close to LT the whole way.  (or, if you're not- you should be) No reason to muck up the works trying to digest something.  Stick with water and sports drink. 

What works just fine at 85% of LT in a training run- feels a little different at 100% LT in a race.

I did this too for my first HM last January and felt fine til the end. =)