GDR2 (General Distribution Release) better known as a MS WP OS update. We're on the second one now, which is where the 2 comes from. They're generally small revisions to the OS. This one isn't looking too special.
The Amber update is Nokia's. It's thought that on Nokia phones GDR2 and Amber may be issued at the same time. The Amber update brings the better smart camera technology to all Nokia phones.
There are people confusing the GDR2 and Amber updates and have a tendency to think of them as being the same. However GDRs are MS and all phones will get it where with Amber only Nokia phones will get that.
Hope that helps. If you do a search on GDR2 you will find the information on it but be aware that some people have mixed up the two updates.