Complex sentence: Galep is the graphic creator of "Tex Willer", whose adventures he drawed for more than 40 years, even if in time other valuable illustrators alternated with him on the pages of this popular comic book.
Split into simpler sentences: Galep is the graphic creator of "Tex Willer", whose adventures he drew for more than 40 years. Other illustrators alternated with him on the pages of this popular comic book.