You can, I think. You should need a total of about 25-30% power vs. Skaven / Berserkers (can include the 7% power skill) and Opportunist on your shield weapon, and you can push Plaguemonks out of their attack.
About his new shield weapon: It’s a really strange weapon, since - even though it has a shield - it’s designed way more for single target fights. The light attack chain does have a bash, but it feels like a different bash from that of other shields. The heavy attack chain also starts with an overhead chop, which actually does very good single target damage in combination with GK’s skills. I could one hit SV on Legend with it if I got a (relatively easy) headshot with it. The push-bash combo is also done differently from other weapons: you push and then light attack really quickly after. It also lacks good cleaving sword sweeps, but has the same push attack into power attack combo as imperial S&S.
What I like about GK is his melee versatility. Because he can pull a shield out of his back pocket, you can cover the weaknesses of other Krub weapons pretty nicely. Things like the Executioner have great damage output, but fold under pressure. In that case you can draw your backup and make space, however. It also alows him to cover multiple roles on the fly. Need to make some space in the frontline? Time for a one man shield wall. Need to hack away at something? No problem, just draw that damage dealer.
That’s why I feel that - ironically - the Brettonian Longsword is not the best pick for GK. That weapon has versatility as a strenght, which is a little less important for GK because of his backup weapon. I actually had most success with him with a combination of imperial S&S and the executioner. The Executioner is the Executioner, off course, and GK’s single target skills + power attack bonus skill really synergize well with it, while still leaving it as a quite decent damage dealer vs. hordes. The imperial S&S covers the Exe’s weaknesses rather nicely because it’s better vs. (mixed) crowds than the brettonian one. GK’s skillset also makes it do a suprisingly respectable damage output. My preverence could also be because those are weapons I’m just more experienced with, however…