Gallery Grid: Hover on image will add a gradient overlay over image

You can use this code to Website Tools > Custom CSS.

/* Gallery hover 07 */
.gallery-grid-item {
    position: relative;
    overflow: hidden;
}
.gallery-grid-item-wrapper::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: #f6004c;
    background-image: linear-gradient(#f6004c 10%, #000);
    opacity: 0; /* Initially hide the gradient */
    transition: opacity 0.3s ease; /* Smooth transition for the gradient appearance */
    z-index: 9999;
}
.gallery-grid-item-wrapper:hover::before {
    opacity: 0.75; /* Show the gradient overlay on hover */
}

Result