Eclipse Oxygen M1 (june) and Grails IDE now work together!
Below are the update site urls required to get the Grails-IDE archive (old!) working in the latest of eclipse IDE versions:
<?xml version="1.0" encoding="UTF-8"?>
<bookmarks>
<site url="http://download.eclipse.org/tools/ajdt/46/dev/update" selected="true" name="AJDT"/>
<site url="http://download.eclipse.org/technology/epp/packages/oxygen" selected="true" name="Eclipse EPP 4.7 (Oxygen)"/>
<site url="jar:file:/opt/bwijsmuller/springsource-tool-suite-3.6.4.RELEASE-e4.5-updatesite.zip!/" selected="true" name="GGTS"/>
<site url="http://dist.springsource.org/snapshot/GRECLIPSE/e4.7/" selected="true" name="GrEclipse"/>
<site url="http://download.eclipse.org/oomph/updates/milestone/latest" selected="true" name="Oomph Latest Milestone"/>
<site url="http://download.eclipse.org/releases/oxygen" selected="true" name="Oxygen"/>
<site url="http://download.eclipse.org/eclipse/updates/4.7" selected="true" name="The Eclipse Project Updates"/>
</bookmarks>