All the times we've seen someone open the gates it just said the gate was opened and they get ready for the next gate if needs be. But do you think the user can control how much the gate opens? For example instead of simply opening the gate completely(100%) can you open it at 50%. I think this would allow the user maintain chakra gate manipulation for longer periods of time which can be important against an opponents of similar strength and speed in which case the fight will be prolonged, so far most the opponents have been completely overwhelmed and the fight quickly ends.
Rock Lee is able to open the 1-5 gates before the time skip, he may be able to open more now after the time skip. Also Lee's respective moves in the 6th,7th and 8th gates they may not be the same as Gai's. It seems to me the user can utilize the speed and strength in different ways and Gai may just wanted to show off the heat from his speed in different ways like a peacock fan, tiger etc..
Kakashi can open at least one gate, as shown during Kakashi's one-handed rock climbing where a shot of his brain shows the relevant point activating, as well as in his usage of the Front Lotus. But its pretty obvious at this point he doesn't use it for combat like Gai does. This is mostly likely due to his low stamina and has a hard time recovering if opened the higher level gates(4 and onwards). Kakashi did also mentioned you need to be "genius(whatever that means)" to open the 5th gate.
Might Guy, who taught Lee the skill, is able to open at least seven. He by the far is the best user and seems to be able to recover very well from the stamina loss and not physically injuring himself. It would be interesting to know what his 8th gate can do. But he would mostly likely die, although we really don't know 100% you would die, it not like we have seen anyone open the eighth gate or has anyone for that matter. Maybe no one really knows what happens when you open the eighth gate. It's possible shinobi assumed given what the 7th gate's drawbacks, they imagined you would most likely die if you opened the 8th gate.
So far there hasn't been any mention as to where or who/clan etc.. started chakra gate manipulation. Gai is haled as the taijutsu specialist of Konoha, and Gai and Rock lee are the only ones who really use this for combat.
But for some reason it seems to me this would be a great addition to Byakugan users. Most Byakugan users are taijutsu specialists anyway thanks to the Byakugan and by far have the greatest chakra control of their tenketsu(also due the byukan), which incidentally the chakra gates control the chakra flow to the tenketsu. It sure would be great to see what the Byakugan users can do with the chakra gates.