If you don't want anybody to hear a gunshot, you use a silencer. Skripals didn't have any bodyguards, one was an older man and the other was a smallish female. So doubtful they could have put up much of a fight (especially if there was a "team of assassins.")But using a gun, would risk being caught, especially if the Skipals were usually out in public places, or at home where neighbours might hear gunshots and see the killers leave or describe their vehicle. They would also have to get a gun through customs or arrange with a contact in the UK.
I guess there are other more messy ways to kill, like stabbing or strangling, but if they wanted to kill both of the them, perhaps not as easy as just leaving some noxious substance somewhere and then taking off before anyone is aware of what is happening. It also spreads fear and uncertainty, which is what they want.
It would have been a lot easier to kill (if that was the goal) using conventional methods. As it is, the method of assassination failed, both Skripals are alive.