A Brazilian Jiu Jitsu Gi and a Karate Gi are very much different. There is a very legit reason why we don’t ever use a karate Gi in Brazilian Jiu Jitsu. Some people learn the hard way and if you are one of them you have suffered ripped Bjj Kimonos, split pants, ripped sleeves etc. In Brazilian Jiu Jitsu you must have a Gi that can withstand the demands that comes with the sport and only a Brazilian Jiu Jitsu Gi can do so.
You must understand that Karate Gis or Tae Kwon Do Gis are specific to that sport. In Karate or Tae Kwon Do for example, your opponent will not grab or pull on your gi so there is no real necessity on reinforcing them. These Gis are usually very thin and flimsy since the sport of Karate or Tae Kwon Do takes a lot of quick movements with their strikes. If there were to be a lot of pulling or grabbing of these Gis they would make them more durable but this is not the case with these sports. These martial arts are designed to teach striking and not grappling.
A Brazilian Jiu Jitsu Gi however is designed specifically for grappling and designed to withstand the pulling and grabbing that Bjj consists of. The Brazilian Jiu Jitsu Gi is made similar to Judo Gis since that martial art also consists of lots of throws and grabbing of the Gi which is why it is made thick as well. If you notice on a Bjj Gi, all the places that are prone to ripping eventually are reinforced with lots of stitching to prevent rips of any sort. A Bjj Gi is also made with very thick collars to make it much harder to tear when your opponent is pulling on it since it is a very common place to pull in Bjj. Another thing about a Bjj Gi is the variety of the unique styles and colors. Although the traditional colors of Bjj kimonos are white and blue, now a days you will see them in all sorts of colors such as red, yellow, camo, green, etc. So again the difference between a Karate Gi and a Bjj is of course the durability. Again if you were to ask if you could use a karate gi in Bjj, well the answer is a simply no.