Hi.
I managed to do this by exporting all of my GNotes notes, writing a program that strips HTML tags then importing the notes using the Import Folders feature. Had to re-create all of the notebooks (Evernote didn't create notebooks out of directories in the Import Folder). Rather laborious but I've done it now! The only niggle is that Evernote uses the first line of the note as the title, and does not remove the first line of the note. A minor irritation. I couldn't find a way around this.
If anyone is interested, here is the code (in bash):
#!/bin/bash
# Must be run in the folder containing the desired gnotes backup
BASE=/tmp/gnotes
rm -Rf $BASE
IFS=$'\n'
for N in `find ./ -maxdepth 1 -mindepth 1 -type d `
do
FOLDER=$N
mkdir -p $BASE/$FOLDER
for SUB in `find $N/ -mindepth 1 -type d`
do
TITLE=`sed 's/.*<p class="note_content">//;s/&/a/g;s/ / /g;s/"/"/g;s/<br>/\n/g;s:</p>.*$::' $SUB/content.html | head -n1|sed 's#/#_#'`
sed 's/.*<p class="note_content">//;s/ / /g;s/&/\&/g;s/>/\>/g;s/</\</g;s/"/"/g;s/<br>/\n/g;s:</p>.*$::' $SUB/content.html > $BASE/$FOLDER/$TITLE.txt
done
done