Hello,
i need help. Simple gallery plugin use rel atrribute but its not valide in w3c validator, can you help me whne I can rename it and how? Thank you

Unfortunately you cannot remove it without breaking the extension. The rel="xxx" extension is used by the lightbox engine to turn the image links into a gallery.

Ok so if I cant remove it is possible to rename it? Use gal="" instead of rel=""???

gal= is not permitted as well.
You can try seeing what values are permitted in the rel attribute and change it accordingly.

PS. You will probably have to hack the lightbox engine as well so always keep a backup of your files in case you update the extension