How do Gai's Gates work? During the Chunin exam,  


   when Lee fights Gaara, Kakashi says that as you open more Gates, the risk of death rises.
 Well, when Guy fights Kisame, Guy has already opened several Gates (up to 6) before the final attack, and they have been open for quite a lot of time (relatively) before the seventh Gate is opened.  
 And of course as he opens more Gates, with his stamina rate supposedly decreasing exponentially, I'm pretty sure he was supposed to be "pretty exhausted".  
 But still, when he opened the seventh Gate, he still had enough time to talk and stop Kisame from escaping.  


Shouldn't this be too much for him?
No, you're not quite right about it. 

There are eight gates at total. Each gate is a special point in the body that limits the flow of chakra. This is done in order for the body to live longer (more chakra flowing = the body is expiring faster). Opening a gate lets chakra flow freely through it, giving the shinobi a boost in his powers, but as a price his body is suffering from that power greatly. 

Each gate gives a boost in a special power, e.g. the first gate removes the restrains on the muscles, the second improves physical strength and re-energizes the body, and so on. 

Now, opening the gates is very dangerous by itself, and it's becoming more dangerous the more gates you open, that's right. The reason for that is that the body simply can't stand the chakra, and gets damaged by it. That's why it's hard to open the gates, and it requires a lot of training to use them. Guy has gone through a lot of training to learn to use that much gates though.

As for the side effects, the seventh gate (which Guy used) causes the user's muscle fibres to be


    ripped to shreds, causing intense pain if anything or anyone touches them


and finally the last, 8th gate gives the user some tremendous power, but costs him his life.

So, to summarize: since Guy has gone through a lot of training to learn to open the gates and use their power, he can use them in the battle. Of course, there are side-effects, even for him, but they usually happen after the battle when he closes the gates once again. Nevertheless, opening the last gate would cost him (or anyone else, for that matter) his life.



source: Naruto Wikia - Eight Gates.



Edit: it's wrong to say that guy doesn't suffer from opening the gates, he does, though not immediately, that's probably why you were confused. It happens in chapter 512 of the manga, after 


   Kisame uses water prison and kills himself.


Here's a screenshot with Guy starting to suffer from the Gate opening (not sure if it's a spoiler, but I'll mark it as one just in case):