They likely dipped into the same sources of inspiration. Miura had a doctorate in art history and took a lot of inspiration from classical artists like Hieronymous Bosch, M.C. Escher, Gustave Dore, Pieter Bruegel, and Frank Frazetta. Wouldn't be surprised if McKenna also borrowed from artist like these.

More about McKenna here [edit: fixed link]. Legend of Zagor was in '93 so the start of Berserk would have beaten it by a few years, although I feel it unlikely that McKenna was drawing influence from manga. I know practically nothing about the lineage of fantasy art but it's probable they shared influences, since apparently McKenna was very young and inspired by other artists for this book. From a blog I found while sourcing:

Few gamebook fans could disagree that Martin McKenna is an exceptional fantasy artist whose work verges on the darker (horror) side of the fantasy spectrum. His work here is as excellent as ever and many of the tougher foes look truly terrifying and awe-inspiring (the Chaos Champion, for example.) It is interesting to note that, in places, McKenna apes others’ work (which he often did in other FFs particularly by drawing influences from Hammer Films) and, whilst there is nothing here lifted from Hammer, the Orc throne room is a literal copy of Iain McCaig’s version from Casket of Souls and the Chaos Champion looks strikingly like the LJN-produced Advanced Dungeons & Dragons action figure called Warduke

There were dozens of books in the FF gamebook series and they used a whole bunch of different artists. The above guy mentions Iain McCaig who also had some slick work (1, 2, also did the famous Deathtrap Dungeon). The one FF book that always stuck in my mind for art was Island of the Undead, apparently illustrated by a guy called Russ Nicholson, but that may be because it crossed a bit too uncomfortably into ugly, spooky territory and was too fucking scary for me as a kid.

Are you flying too close to the sun, my friend