To use a gradient background on a section, you have two options:
Export a PNG of the complete gradient at a decent size (like 1280px x 720px) then scale the image fullscreen. Since gradients shouldn't really lose their quality, this should look fine.
Give your section a class in the edit popup, then use a CSS gradient generator to get the CSS for your gradient. Then you can paste the CSS in the Custom CSS section. Example of CSS:
.... here goes your gradient generator code.
Guide: How to Use a Gradient Background on a Section
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