Note that "gonna" is very informal, and you should use "going to"... If you didn't know that, then you will have better luck searching now.

    You know what would probably happen?

Right now if you dropped your coffee, it would spill.  So if someone says, "(Do) You know what would probably happen?

The person you are speaking to knows you are talking about what would happen if you dropped the coffee.

    You know what was probably gonna happen?

The second construction, though, is rare.  Maybe you almost get caught smoking cigarettes, but you and your friend manage to sneak away. Then you might talk about what "was going to" happen.  But the sentence clearly means that it no longer is likely to happen, because of the past tense "was".  Something has changed since then.