a gradient image is the way to go - you can set the form background in form properties (on the format tab) to an image.
There are plenty of free tools out there you create your own images - even use Word or Excel to create the gradient and take a screenshot of it - or use the button in Access per the next paragraph.
Within Access, options are limited. The only control where you can apply gradients is a button which you could use as a background (send to back and locked and sized to fit the form) but probably not a good idea - better to make a temporary big one, get your gradient design and screenshot it