Because GNU License doesn't say it clearly, can i use Drupal to build sites and then sell them and if yes what are my legal restrictions?
Yes you can. You have no restrictions, other than GPL.
You can not encrypt the code, nor can you remove other peoples Copyright info.
The entity you sell the code has the same rights you do.
Thanks for your response. So if i use others' themes or modules i should also add their copyright info without encrypting their code?
You cannot encrypt any portion of the code.
Just leave it all alone.
Once you give your code to a second party, they have the same rights you do. This is why no one sells modules in the Drupal world.
"If I use a piece of software that has been obtained under the GNU GPL, am I allowed to modify the original code into a new program, then distribute and sell that new program commercially?"
See all hot content.
Drupal is a registered trademark of Dries Buytaert.