To set different height for desktop and mobile with a Gallery Slideshow, you can do these
#1. Slideshow Simple/Full
First, you need to set height on Desktop

Next, find ID of Slideshow section
In my example, we will have: section[data-section-id=”66ed2c3e2f063471736e09c1″]

Next, use this code to Custom CSS to change height on Mobile
/* Mobile Slideshow */
@media screen and (max-width:767px) {
section[data-section-id="66ed2c3e2f063471736e09c1"] .gallery-fullscreen-slideshow {
height: 35vh !important;
}
}

We will have

#2. Slideshow Reel
First, you need to set Reel Height for desktop

Next, find Reel ID.
In my example, we will have: section[data-section-id=”6707a04e0b698b4fb4bd8f3e”]

Next, use CSS like this to Custom CSS Box
@media screen and (max-width:767px) {
section[data-section-id="6707a04e0b698b4fb4bd8f3e"] .gallery-reel {
height: 35vh !important;
}
}

Result
