Bubblegum Crash is the conclusion to Bubble Gum Crisis. The premise is that the city has become a better place to live and the Knight Sabers are not needed at all now. Each of the original members have new careers and are enjoying life for once, however as you would expect this does not last long and soon the Knight Sabers have to reform and battle mad boomers.
Bubblegum Crash does offer more story than Bubblegum Crisis but does come across as just another 3 episodes. Apparently the story goes that the Crash episodes would have been part of the Crisis series, but problems arose and we have the current situation. Some changes have been made to the hardsuits, and the lingerie van has been redesigned and works much better.
What Crash does do is create a better picture of the world of the Knight Sabers. The A.D. police get a bigger screen time with Leon spouting off about life and general feelings and opinions on the Knight Sabers and the current boomer situation. The real downside to Crash is the fact that even these extra episodes seem compressed and the writers really needed more time or maybe a fouth episode.
So to sum up these extra final episodes are great, but the story is still bitty and incomplete. I think if Bubblegum Crisis, Bubblegum Crash and Bubblegum Crisis Tokyo 2040 were combined the result would be awesome. That is not taking anything away from the Bubblegum series, its just a shame that one has the story, one has the scope, and one has the best character representation.
Make of that what you will.