Why are "g" tags appearing in the WYSIWYG editor?

If you notice that <g> tags are being inserted into your WYSIWYG's source code, for example:

<g class="gr_ gr_00 gr_alert gr_gramm gr_spell" data_gr_id="00" id="00">Here is some content.</g>

The likely culprit is the Grammarly browser extension. You can remove those tags from the WYSIWYG and temporarily disable the Grammarly extension in your browser to ensure they're not re-inserted.