Hi Ben,
1. It’s not related to GB, as the WP query block will behave the same with or without GB container wrapping it.
2. Firefox might read the style added by the query block differently than the other blocks.
3. You can give this CSS a try, the default WP query block has a larger gap between posts than the width reduced for individual post, this CSS can force the flex parent to use the same value for gap.
ul.is-flex-container.columns-4.wp-block-post-template {
gap: 0.9375em;
}
Let me know if it works.