flexbox gap

1gap: 10px 20px; /* ⇳ and ⬄ */
2
3row-gap: 10px; /* ⇳ only */
4column-gap: 20px; /* ⬄ only */

1.upper
2{
3  margin:30px;
4  display:flex;
5  flex-direction:row;
6  width:300px;
7  height:80px;
8  border:1px red solid;
9
10  padding:5px; /* this */
11}
12
13.upper > div
14{
15  flex:1 1 auto;
16  border:1px red solid;
17  text-align:center;
18
19  margin:5px;  /* and that, will result in a 10px gap */
20}
21
22.upper.mc /* multicol test */
23{flex-direction:column;flex-wrap:wrap;width:200px;height:200px;}

1div{
2    	column-count: 3;
3        column-gap: 30px;
4        background: yellow;
5}

1/**
2Flexbox doesn't have collapsing margins.
3Flexbox doesn't have anything akin to border-spacing for tables (except for CSS property gap which isn't supported in Safari, caniuse)
4Therefore achieving what you are asking for is a bit more difficult.
5
6In my experience, the "cleanest" way that doesn't use :first-child/:last-child and works without any modification on flex-wrap:wrap is to set padding:5px on the container and margin:5px on the children. That will produce a 10px gap between each child and between each child and their parent.
7**/
8.upper
9{
10  margin:30px;
11  display:flex;
12  flex-direction:row;
13  width:300px;
14  height:80px;
15  border:1px red solid;
16
17  padding:5px; /* this */
18}
19
20.upper > div
21{
22  flex:1 1 auto;
23  border:1px red solid;
24  text-align:center;
25
26  margin:5px;  /* and that, will result in a 10px gap */
27}
28
29.upper.mc /* multicol test */
30{flex-direction:column;flex-wrap:wrap;width:200px;height:200px;}
31<div class="upper">
32  <div>aaa<br/>aaa</div>
33  <div>aaa</div>
34  <div>aaa<br/>aaa</div>
35  <div>aaa<br/>aaa<br/>aaa</div>
36  <div>aaa</div>
37  <div>aaa</div>
38</div>
39
40<div class="upper mc">
41  <div>aaa<br/>aaa</div>
42  <div>aaa</div>
43  <div>aaa<br/>aaa</div>
44  <div>aaa<br/>aaa<br/>aaa</div>
45  <div>aaa</div>
46  <div>aaa</div>
47</div>