afaik, there are 2 versions of 3.0.12. One with all the WTP in there, one without. If you used the latter, you have to install WTP dependencies yourself
I recommend you to move to the gpd 3.0.13. The error you refer to is a known problem in 3.0.12 that was fixed.
i have used gpd 3.0.13 as you have said.
but during installing i am getting a new error like the
"selected archive does not contain the update site"
what to do with this?
Follow the instructions in the readme of the archive.