On width 600px the 2 columns fall under each other and create an empty space on the right. But they should fill the whole space. This issue is the same with more than 2 columns.
To Reproduce

Possible fix
wp-block-column is using margins for gaps:

Using paddings will fix this issue.
Possibly related: #12640.
Yep, all sort of related I think:
https://github.com/WordPress/gutenberg/pull/12199
https://github.com/WordPress/gutenberg/issues/12640
https://github.com/WordPress/gutenberg/pull/12408
@drdogbot7 has a couple nice codepens of the issue in https://github.com/WordPress/gutenberg/pull/12408
The Wordpress 5.1 is out but this issue still remains...
@IvanPr This kind of comments on multiple issues don't really help. If you want to move things forward, maybe you could take a stab at these?
Sure, here are the CSS rules that solved an issue for me:
.has-3-columns > .wp-block-column:last-child {
margin-right: 0;
}
@media (max-width: 781px) {
.has-2-columns > .wp-block-column,
.has-3-columns > .wp-block-column,
.has-4-columns > .wp-block-column {
flex-basis: calc(50% - 32px);
flex-grow: 0;
}
.has-4-columns > .wp-block-column:nth-child(odd) {
margin-left: 0;
margin-right: 32px;
}
.has-4-columns > .wp-block-column:nth-child(even) {
margin-right: 0;
}
.has-3-columns {
flex-flow: row wrap;
}
.has-3-columns .wp-block-column:first-child {
flex-basis: 100%;
margin-right: 0;
}
.has-3-columns > .wp-block-column:nth-child(even) {
margin-left: 0;
margin-right: 32px;
}
.has-3-columns > .wp-block-column:nth-child(odd) {
margin-right: 0;
}
}
@media (max-width: 599px) {
.has-2-columns > .wp-block-column,
.has-3-columns > .wp-block-column,
.has-4-columns > .wp-block-column {
flex-basis: 100%;
margin-left: 0 !important;
margin-right: 0 !important;
}
}
This seems fixed in my testing.
Most helpful comment
Sure, here are the CSS rules that solved an issue for me: