Flex Slider, GNU

wordpress

#1

Hello!
I make wp theme for sale on themeforest. This theme will have license “GNU General Public License v2 or later”. I want use (slightly modifying) Flex Slider (https://www.woothemes.com/flexslider/). This slider have licence “GNU GENERAL PUBLIC LICENSE Version 2”.
Can I modify this slider? Can I use this modified slider in WordPress theme for sale?