Wrong way my friends, wrong way! There is no gnuTIL, no broad spectrum open source software (don't feel bad, there is no proprietary tool covering completely ITIL either).
This been said, you can find hundreds of open source tools that can be used as part of your ITIL processes on availability management, capacity planning, incident management...
Like for all open source projects, some are reliable and function rich, and some are just embryonal projects.
With ITIL v3 that is now offering a better cover of your company's activities (especially design and development), I'd recommend to use ITIL on your ITIL implementation: create a portfolio of open source tools and enhance them with an ITIL approach.
This way, you can experiment ITIL on an internal project, create personalized tools to support your upcoming ITIL processes, and last but not least, contribute to the open source community.
Good Karma guaranteed!
