If none of the available gateway providers will work for you the checkout would need to be custom to work with eCart. One way you could do this would be to create the checkout for another gateway using the checkout wizard. You could then go back to the checkout and confirm pages and update them to work with your gateway.
This will require custom hand coding though and is not something I would suggest unless you are experienced in this type of thing. The idea is that you would use some documentation from your gateway provider to craft the code needed, wherever you need to make use of values from the cart, like the item price or the total, you would use the eCart bindings for these values.