Transmission leak

Not so much a leak as it was a gusher. Was reversing out of driveway and it was barely moving. Then I notice a large pool of fluid. It gushed out about a quart.
I got it up on stands and looked around hoping that it was just a cooling line that broke or popped off but it appears to be coming from the transmission housing.
See attached pic. In addition to the drips on the outside of the housing, looking through the inspection hole I could also see drips.

What does this mean? What is this likely to be?
I have a '99 328i with the automatic GM tranny.

Sent from my EVO

Could be the filter housing gasket. Can't tell from pics though.

I've been reading around and found people talking about large leaks out of a breather hole for the transmission. And, those people usually associated it with cold weather. Same as my situation.

So, what causes the fluid to gush out of the breather hole?

Here's an update.
I did a fluid and filter change. After changing the fluid I continued running it through the gears while up on the stands. And boy it sounds bad. Lots of grinding and slipping. It wasn't on the ground and of course not under any real load and I couldn't even give it any gas because it would just slip and grind...both in drive and reverse.

The transmission/limp mode light (the one with the triangle and exclamation point) kept flashing when I tried to give it any gas.

Unless someone thinks it could be anything else, I think I'm in the market for a rebuilt transmission. Just surprised that is have such a serious failure with no symptoms. It had been driving strong and shifting smoothly.

Anyone know where I can get a transmission near Columbus, Ohio? I might be up for trying it myself but I may use a shop. Any good ones in the area?

Sent from my EVO