Mukund,
The DSL604T source is available, but for DSL502, I did not find any. Further DSL502 has few binary files in flash for which no source is available. These files are basically CGI files, used to update flash data. I am not sure if D-link India guys ever heard of GPL.
I think DLink doesnt need to open source for the CGI Files, but overall they should relase source code of the based operating system and all other GNU - GPLed components they are using, including the kernel.
Thanks & Regards, Mitul Limbani, Founder & CEO, Enterux Solutions, The Enterprise Linux Company (TM), www.enterux.com