I’ve read that gunpowder artillery was used during the War of the Roses. Since the War of the Five Kings draws so much inspiration from the Wars of the Roses, I was wondering: would one or multiple sides having access to gunpowder artillery have made a major difference in the War of the Five Kings?

Gunpowder artillery was used in the Wars of the Roses. Not always successfully – Warwick had cannons at the 2nd Battle of St Albans, but dampness meant that the powder wouldn’t fire, which rendered them useless. Likewise, at Northampton, Buckingham had cannons against Warwick, but rain knocked them out of action before the fighting started. 

Even when they weren’t, they usually weren’t the significant factor – Edward IV fired only a single barrage of cannons at Losecoat FIeld, but given that the rebels routed even before his charge could hit home, the battle was decided by their lack of discipline. Cannons were used at Barnet, but Warwick’s cannons overshot their target, the fog rendered artillery ineffective, and the deciding factor was a friendly fire incident that led two different Lancastrian units to attack each other and cause a panic. Cannons were used at Tewksbury, but the main deciding factor of the battle were 200 Yorkist spearmen hidden in the woods who attacked the Lancastrian rear and flank, causing a rout. Cannons were used at Bosworth as well, but the primary factor was the betrayal of the Percys and Stanleys. 

In the field, therefore, I wouldn’t expect cannons to make much of a difference. To me, the main difference would seem to be in sieges. Most of the Westerosi castles aren’t built to withstand cannon fire – Casterly Rock and the Eyrie would be ok, but King’s Landing, Riverrun, etc. would have had their walls smashed open.