Use this code...
div.sprocket-features.layout-slideshow .sprocket-features-img-container::before {
background: rgba(0, 0, 0, 0) linear-gradient(rgba(186, 255, 186, 0), #bababa);
}
To come up with the full gradient code use this tool
http://www.colorzilla.com/gradient-editor/
.
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.