Use this code...
.g-slider .g-slider-slides .g-slider-slides-slide .g-slider-slides-image::after {
content: none;
}
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.