To achieve 2 columns of images Gallery on Desktop and 1 column on Mobile, you can follow these.
#1. Gallery Grid
You can hover Gallery Section > Click Edit Section

Change number of column images on Desktop here

On Mobile, you can use this code to Website Tools > Custom CSS to force it to 1 column.
@media screen and (max-width:767px) {
.gallery-grid-wrapper {
grid-template-columns: repeat(1,1fr) !important;
}}

If you want to apply this for specific gallery, you can find Gallery ID. Use this tool.
In my example, it is:
section[data-section-id="66c2fd5aa1b5905d9100c36c"]

Next, use this CSS code
section[data-section-id="66c2fd5aa1b5905d9100c36c"] {
@media screen and (max-width:767px) {
.gallery-grid-wrapper {
grid-template-columns: repeat(1,1fr) !important;
}}}

#2. Gallery Strips
With Gallery Strips, you can Edit Section

Then adjust the Row Height. For each Row Height value, the gallery will display a different number of columns of images.

With this change, Strips is already 1 column on Mobile, if you want to force Strips to 2 columns on Mobile, you can use this CSS code to Website Tools > Custom CSS.
@media (max-width: 767px) {
.gallery-strips .gallery-strips-wrapper {
columns: 2;
column-gap: 0;
height: auto!important;
display: block!important;
padding: 0 5px;
}
.gallery-strips-item-wrapper {
height: auto!important;
}
.gallery-strips-item {
position: relative!important;
transform: none!important;
width: 100%!important;
display: block;
padding: 5px!important;
box-sizing: border-box;
}
.gallery-strips .gallery-strips-item img {
height: 100%!important;
width: 100%!important;
}
}

To make it affect on specific Strips only, you can find Gallery Strips ID, similar #1, then use CSS code like this.
section[data-section-id="66c2fe1078d02d100042a466"] {
@media (max-width: 767px) {
.gallery-strips .gallery-strips-wrapper {
columns: 2;
column-gap: 0;
height: auto!important;
display: block!important;
padding: 0 5px;
}
.gallery-strips-item-wrapper {
height: auto!important;
}
.gallery-strips-item {
position: relative!important;
transform: none!important;
width: 100%!important;
display: block;
padding: 5px!important;
box-sizing: border-box;
}
.gallery-strips .gallery-strips-item img {
height: 100%!important;
width: 100%!important;
}
}}

#3. Gallery Masonry
You can edit the Gallery Masonry Section then change this option

To make it to 1 column on mobile, you can use this CSS code
@media screen and (max-width:767px) {
.gallery-masonry-wrapper.gallery-masonry-list--ready {
height: auto !important;
display: grid;
grid-template-columns: repeat(1,1fr) !important;
grid-column-gap: 10px;
grid-row-gap: 10px;
}
figure.gallery-masonry-item {
position: relative !important;
width: 100% !important;
transform: unset !important;
}
.gallery-masonry-item-wrapper {
height: 100% !important;
}
.gallery-masonry-item-wrapper img {
width: 100% !important;
}
}

to make it run on specific Masonry, you can find Masonry Section ID, use #1 tool. Then use CSS code like this.
section[data-section-id="66c2fedf4f347e205755537a"] {
@media screen and (max-width:767px) {
.gallery-masonry-wrapper.gallery-masonry-list--ready {
height: auto !important;
display: grid;
grid-template-columns: repeat(1,1fr) !important;
grid-column-gap: 10px;
grid-row-gap: 10px;
}
figure.gallery-masonry-item {
position: relative !important;
width: 100% !important;
transform: unset !important;
}
.gallery-masonry-item-wrapper {
height: 100% !important;
}
.gallery-masonry-item-wrapper img {
width: 100% !important;
}
}}
