Can reuse theme stuffs with GNU license?

I downloaded some themes from internet. Most of them are free and having attachment license.txt (GNU GENERAL PUBLIC LICENSE ). Of course, I must have new ideas, new layout, new images… how about code? No one can prove that they have license with any code line. Because everything comply with rules of language (PHP, JS …). Well, I can reuse coding for commercial theme?

Hi Tarzan, With GNU license, you can modify or use any part of code in your commercial products. Just one thing to remember: your products must be open source. FYI : http://vi.wikipedia.org/wiki/Gi%E1%BA%A5y_ph%C3%A9p_C%C3%B4ng_c%E1%BB%99ng_GNU

MoJs said
Hi Tarzan, With GNU license, you can modify or use any part of code in your commercial products. Just one thing to remember: your products must be open source. FYI : http://vi.wikipedia.org/wiki/Gi%E1%BA%A5y_ph%C3%A9p_C%C3%B4ng_c%E1%BB%99ng_GNU

Thanks for your reply (and vn link)