This page provides a bundled P2 repository and different SDK dropins (in runnable form) for each build.
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
All downloads are provided under the terms and conditions of the Eclipse Foundation Software User Agreement unless otherwise specified.