This claim is misleading because while Gigi Hadid and Bradley Cooper are walking together, there has not been a vocal statement from either party confirming a relationship. The picture is framing them as a couple.
This Twitter post by PopBase acknowledges that they are rumors which lets the reader know the relationship has yet to be confirmed.
PopBase seems reliable because the content they post is not opinionated. They seem to be reporting on news that has already been reported on such as award show winners, celebrity Instagram posts, etc. There is no author accredited for the posts.
Harper's Bazaar published an article eight days ago that lays out Hadid and Bradley's relationship timeline. The author, Rosa Sanchez, who is a senior news editor at Harper's Bazaar and previously a news editor at ABC News, seems to be a reliable source. She discusses each time the pair have been spotted together with solid proof from paparazzi photos.
On November 1st, for example, Sanchez reports that the pair were seen attending an Off Broadway play together.
The photo tweeted by PopBase is from a public outing when the pair were in NYC.
There are several articles that argue Hadid and Cooper are dating, however this claim has yet to be 100% confirmed. The two have not been seen in a romantic exchange, so their relationship can only be confirmed as platonic for now.
Sources: https://www.harpersbazaar.com/author/239085/rosa-sanchez/
https://www.harpersbazaar.com/celebrity/latest/a45721960/gigi-hadid-bradley-cooper-relationship-timeline-photos/