Don't use "gonna." Use "going to" when you write, and use "going to" in your mind when you think. It will naturally shorten itself to something like "gonna" when you speak if you speak quickly. "I'm going to send it tomorrow" is correct. It conveys the idea of a single event that is going to happen at an instant of time. You're thinking of "send" as something simple that doesn't take any time. The letter goes into the mailbox at 1:56 p.m. Clunk! "I'm going to be sending it tomorrow" is correct. It conveys the idea of a process that lasts for more than an instant. You are looking at it more closely. Tomorrow, I will print it out at 1:12, fold it and put it into the envelope at 1:23, click the flap and glue it at 1:24, write the address on it at 1:33, walk out the door at 1:42, arrive at the mailbox at 1:56. Clunk! "I'm going to be sending it tomorrow," because it is an extended process that starts at 1:12 and ends at 1:56. Despite my long explanation, there is very, very little difference in meaning. Unless you need to show you know it for an exam, when you speak you can just use the first form. When you listen, the important facts will come from other parts of the sentence.

"gonna" is the contraction of "going to". but "gonna" represent an informal speech or casual conversation.