Here's an example, change the from / fromName / to entries in the gel:email tag first though:
<gel:script
xmlns:gel="jelly:com.niku.union.gel.GELTagLibrary"
xmlns:util="jelly:util"
>
<gel:parse var="testxml">
<someroot>
<somexml>hello, world!</somexml>
</someroot>
</gel:parse>
<gel:set asString="true" select="$testxml" var="testxml2" />
<util:replace new="&lt;" old="<" value="${testxml2}" var="testxml3" />
<gel:email from="blah" fromName="blah" to="blah" subject="HTML test">
Testing html format email:<br>
<div style="white-space: pre">
${testxml3}
</div>
</gel:email>
</gel:script>
Results in:
Testing html format email:
<?xml version="1.0" encoding="UTF-8"?>
<someroot>
<somexml>hello, world!</somexml>
</someroot>