Hi. Due to my limited understanding, I asked pretty much the same question in this forum. I got a number of replies that have fixed my issues. Thank you.
This is the css I put in the customiser. It removes the right margin space and forces single column in mobile view. Quite elegant.
.gallery-item { margin-right: 4%; padding-right: 0; width: 48%;}
.gallery-item:nth-child( 2n ) { margin-right: 0; }
@media screen and (min-width: 30em) {
.gallery-columns-3 .gallery-item { padding-right: 0; width: 32%; margin-right: 2%; }
.gallery-columns-3 .gallery-item:nth-child( 3n ) { margin-right: 0; }
}
@media screen and (max-width: 30em) {
.gallery-columns-3 .gallery-item {
min-width: 100%;
}
}