Gerhard is the background artist, who does pencils, inks and colors. His first comic work with Dave was the Epic Illustrated short story His First Fifth. He started on the regular issues with issue 65, but his first credit is in issue 86.
Dave Speaks on 'What Ger Does'
From the Note From The President from issue 131:
Oh right! Something I've been meaning to mention. A lot of people have been asking what Gerhard does on the book since he was nominated for "best inker" in the Harvey awards last year. Gerhard does all of the backgrounds in pencil and ink on the covers and interior pages. I write the words, letter the balloons (pencil and ink) and pencil and ink all of the characters and all of the characters' clothing. Gerhard puts the mechanical tones on the backgrounds where he sees fit and puts the tones on Cerebus (and the costumes if I indicate it, except for Jaka's skirt on page 12-13 in this issue. I put the tone on that. I don't know why. Just felt like it, I guess).
So, yes Gerhard inks his backgrounds, but he also pencils them and I'd say he's at least as good a background pencil artist as he is a background ink artist but I never vote in the awards so what do I know, right? If I have a specific background or angle in mind, I'll suggest it to him or do a rough sketch on the page. It's up to him if he wants to follow it or do something else. As we pointed out at the Northampton Summit if Gerhard wants to draw the interior of a submarine instead of Pud's tavern, that's up to Gerhard; that's what creative freedom is about. He draws Pud's tavern because he wants the story to work the same as I do. If I ask for advice on what I'm doing he gives it and I take it or leave it. If he asks for advice I give it and he takes it or leaves it. Creative Freedom. Rah rah. Oh, and Gerhard colours the covers, too with Dr. Martin dyes.