My Pit Bull has a growth in her mouth. Is it cancer or infection?

That does look concerning. Unfortunately there is no way to tell from the appearance alone as to whether or not it cancerous. You are correct in that it may be a benign growth, it may cancer, or it may some sort of inflammatory mass from an infection of the gums either due to a dental problem or due to an embedded bit of small stick or something along those lines. Gingival hyperplasia (overactive growth of the gum tissue) is possible, however it usually appears less "mass-like."
If she is still eating, active and happy, make an appointment with your vet at your convenience in the coming weeks to have it checked out. If she is not eating or otherwise seems ill, or if you are noticing that her breath is really bad, get her in as soon as you can.
Your vet may recommend that a biopsy be considered. A biopsy is the only way to determine what exactly it is, how dangerous it is, and what needs to be done about it.
I hope thats helpful for you! Thanks for using Petco Pet Education Center, formerly Petcoach!