You can't keep the gumballs in your inventory.
It takes a few seconds for the gumball to come from the machine, it always shows a red gumball coming down the pipe.
You sometimes get a moodlet but not always, I tried about 10 times before I got it.
If you have a grumpy or rebellious sim, then you can 'kick it' but the gumballs don't always come out that way.
And sometimes you'll get a message saying that your sim got 2 gumballs, or something like that.
I'll post some pictures soon.