Gaga is a pop artist, Rihanna is a (mainly) pop artist with a R&B/Reggae background, and Adele... well I have 21 listed under 'R&B/Soul'. Even though she has pop influences in her music, I personally think the soul components overpower the pop ones.
People don't get that "Pop" is an umbrella term. It's something lots of artists are. Adele is just as poppy as Lady Gaga, but a lot of people think Lady Gaga is more pop because "dancepop" has been popular for a longass time.