I'm using Prince 15.4.1 and if I copy the code from https://developer.mozilla.org/en-US/docs/Web/CSS/column-gap:

<style>
    .flexbox {
        display: flex;
        flex-flow: row wrap;
        height: 100px;
        column-gap: 20px;
    }

    .flexbox > div {
        border: 1px solid green;
        background-color: lime;
        flex: 200px;
    }

    div:nth-of-type(3n) {
        flex: 300px;
    }
</style>

<div class="flexbox">
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
</div>

Instead of be like this:



it's reproduced like this:

This should be supported in the latest builds and will be included in Prince 16.