None of the code I found on support forums for centering Gravity Forms submit buttons worked, so I had to write my own. The code below is for a specific form and only applies to that form ID. You'll have to replace the form ID number in the code with your own.

#gform_submit_button_9 {
background-color: #ffd40d;
color: #000000;
display: flex;
align-items: center;
justify-content: center;
width: 50% !important;