1 - Create this bookmarklet:
Code:
javascript:window.location='x-omnifocus://newtask?name='+encodeURIComponent(document.title)+'¬e='+encodeURIComponent(window.location+'\n\n')+encodeURIComponent(getSelection())+'&quickentry=0';
2 - Unzip the attached file
3 - Click on it the bookmarklet and when it asks which application do you want to use to open omnifocus:// choose the script "OmniFocus URI Handler"
http://forums.omnigroup.com/showthread.php?t=8251
4 - Enable "Title tweaks" in gmail > options > labs so the title of the task it's cleaner