The Gunners want to recruit the Raya to give Aaron Ramsdale some competition for the number 1 spot. Raya has 1 year left on his current deal, and has stalled on signing an extension, so the Bee's have no option than to sell, however they won't be pressured into selling for a low price.
Brentford's manager, Thomas Frank has this to say on David Raya:
“He's worth £40m. He was one of the four best keepers in the PL last season… What is the fair price for a player with one year left? Declan Rice went to Arsenal for £105m? And Harry Kane, they're talking about £150m?”

David Raya
Aaron Ramsdale in the Premier League last season:
Saves - 154 / 94
Save success rate - 75.1% / 69.3%
️ Clean sheets - 12 / 14
Accurate long balls - 410 / 138
Goals conceded - 46 / 43
Errors leading to goal - 1 / 2