Gaga at her peak was more global, even her least successful album debuted with more copies WW than Rihanna's most successful.
You don't lose your global status with one underperforming album. Unless you think people that have bought Gaga's music, streamed ha, heard of ha on the news and so on suddenly deleted her from their memories ?
Gaga has played stadiums in every continent, she has had a smash single and album in more countries than Rihanna and Gaga was named once the most famous person in the world by Guiness in 2012 and the most powerful person in the world in 2011 by Forbes and 2014 by Guiness, something Rihanna has yet to achieve.
Gaga's most successful album sold more WW than Rihanna's 2 most successful albums combined.
Gaga's touring gross outside the USA is more than Rihanna's overall gross including the USA and so on.
It's really not up for debate.