As an example,
<gallery caption="123"> File:Test.png|456 File:Test.jpg|678 </gallery>
Currently, galleries look like as follows,
<ul class="gallery mw-gallery-traditional" typeof="mw:Extension/gallery" data-mw='{"name":"gallery","attrs":{},"body":{}}'>
<li class="gallerycaption">123</li>
<li class="gallerybox">
<div class="thumb"><figure-inline typeof="mw:Image"><a href="./File:Test.png"><img resource="./File:Test.png" src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/95px-PNG_Test.png" data-file-width="3120" data-file-height="3920" data-file-type="bitmap" height="120" width="95" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/143px-PNG_Test.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/190px-PNG_Test.png 2x"/></a></figure-inline></div>
<div class="gallerytext">456</div>
</li>
<li class="gallerybox">
<div class="thumb"><figure-inline typeof="mw:Image"><a href="./File:Test.jpg"><img resource="./File:Test.jpg" src="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/95px-JPG_Test.jpg" data-file-width="3120" data-file-height="3920" data-file-type="bitmap" height="120" width="95" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/143px-JPG_Test.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/190px-JPG_Test.jpg 2x"/></a></figure-inline></div>
<div class="gallerytext">678</div>
</li>
</ul>In that structure, the media are inline and maybe the whole thing can be considered a figure. To play around with that idea a little, it could look like,
<figure typeof="mw:Extension/gallery" data-mw='{"name":"gallery","attrs":{},"body":{}}'>
<ul class="gallery mw-gallery-traditional">
<li class="gallerybox">
<div class="thumb"><figure-inline typeof="mw:Image"><a href="./File:Test.png"><img resource="./File:Test.png" src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/95px-PNG_Test.png" data-file-width="3120" data-file-height="3920" data-file-type="bitmap" height="120" width="95" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/143px-PNG_Test.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/190px-PNG_Test.png 2x"/></a></figure-inline></div>
<div class="gallerytext">456</div>
</li>
<li class="gallerybox">
<div class="thumb"><figure-inline typeof="mw:Image"><a href="./File:Test.jpg"><img resource="./File:Test.jpg" src="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/95px-JPG_Test.jpg" data-file-width="3120" data-file-height="3920" data-file-type="bitmap" height="120" width="95" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/143px-JPG_Test.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/190px-JPG_Test.jpg 2x"/></a></figure-inline></div>
<div class="gallerytext">678</div>
</li>
</ul>
<figcaption class="gallerycaption">123</figcaption>
</figure>However, the outcome of the RFC (T251641#6212416) to switch from <figure-inline> to <span> for inline media was,
with the condition that we switch to <figure>s in galleries
The argument was that galleries can be considered presentational and structured as a list of figures. That would be,
<ul class="gallery mw-gallery-traditional" typeof="mw:Extension/gallery" data-mw='{"name":"gallery","attrs":{},"body":{}}'>
<li class="gallerycaption">123</li>
<li class="gallerybox">
<figure typeof="mw:Image"><a href="./File:Test.png"><img resource="./File:Test.png" src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/95px-PNG_Test.png" data-file-width="3120" data-file-height="3920" data-file-type="bitmap" height="120" width="95" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/143px-PNG_Test.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6a/PNG_Test.png/190px-PNG_Test.png 2x"/></a>
<figcaption class="gallerytext">456</figcaption>
</figure>
</li>
<li class="gallerybox">
<figure typeof="mw:Image"><a href="./File:Test.jpg"><img resource="./File:Test.jpg" src="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/95px-JPG_Test.jpg" data-file-width="3120" data-file-height="3920" data-file-type="bitmap" height="120" width="95" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/143px-JPG_Test.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/28/JPG_Test.jpg/190px-JPG_Test.jpg 2x"/></a>
<figcaption class="gallerytext">678</figcaption>
</figure>
</li>
</ul>The first list item being the gallery caption (<li class="gallerycaption">123</li>) is a little distasteful though.