Older GMF Notation releases have been moved to archive.eclipse.org, and can be accessed here:
In the Helios release, the GMF project was restructured into the GMF Tooling, GMF Runtime and GMF Notation projects under the Graphical Modeling Project.
A table of previous GMF releases can be found HERE
If you have problems downloading the drops, contact the webmaster.
All downloads are provided under the terms and conditions of the Eclipse Foundation Software User Agreement unless otherwise specified.