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?”