Get to vs. got to be doing sth

Hi!

I have heard people use both of the following expressions: I get to / got to be doing something, e.g. I get to be going now / I got to be going now. Is there any consistency in the usage, e.g. or is it only a matter of preference? Which one is the most common?

I gotta go now = I must go now
I got to be going now = same as above, but sounds even more awckword
*I get to be going now = gibberish

I've got to be going now