Okay, I'll just post some of the differences that occur in the manga and anime, I found these out by sailing around the internet and translating some manga reviews, and I even managed to find out some things by speaking to a few Japanese people...although they told me that the manga was very different to the anime.
But I'll list the changes here;
Gregory House...the apartment building?!
The first notable change for the manga...is the fact that Gregory House is actually an apartment building ( instead of a hotel, like in the series) for people to take up residence in.
Although it still has the look of a european mansion ( as far as I know) and is also pretty old.
Edit: I actually found some screenshots from the manga that someone had posted in a video, and Gregory House still looks like a mansion.
In the real world?
Another change is that the 'apartment' appears to actually be in the real world; whereas in the anime ( and game) the hotel was between the borders of reality and fantasy.
The only proof I have of this, is that the estate agent recommended 'Gregory House' to Tooru...unless of course, the Estate Agent was actually the First Guest.
Catherine: Blood extractor...and organ repo woman?!
In the manga, it would seem that Catherine doesn't just suck peoples blood ( like in the game and anime) with her needle, but she extracts their internal organs as well! O.O; ( all the more reason to be afraid of her XD; )
Nekozombie; Not the hero?
Although I have yet to see this confirmed, Nekozombie may or may not help out the human guest in the manga version of GHS.
In fact, from what I translated ( whether it was a good translation or not) he seems to come across as selfish and rather keen on eating Tooru's goldfish.
And also; according to a translation I got, Nekozombie and Gregory's relationship is 'normal' in the manga version...so they still hate one another, I think. XD
Edit: Once again based on the translation I got, Nekozombie is given as perhaps not being as nice as people thought he was in the anime.
He is not confined to a cell in the manga and apparently walks freely around the hotel.
He also takes delight in terrorizing Tooru's pet goldfish ( apparently) although he may or may not still warn Tooru.
Notible Size Differences.
In the anime of GHS, the guest characters were always more or less the same size as Gregory...or they appeared to be at least.
In the manga of GHS, Tooru and Kimiko are taller than Gregory ( much taller!) which may make them taller than a lot of the other guests too.
However, I'd have to see actual manga scans before I could confirm this difference fully.
Edit: I have seen some manga scans, and I still can't tell. ^^;
Yes, apparently in the manga; Haniwa ( a gang of Haniwa in fact) make an appearance as a mob of gangsters...I just translated this from somewhere, so I don't really have any other details from it.
It just surprised me though...especially when the anime's Haniwa comes to mind; it certainly is a difference from the depressed, unlucky, overworking salaryman. XD;
No gun for Gregs?
Again, this is something I'm not too sure about...but in the manga, Gregory doesn't have the trumpet gun ( a.k.a the Blunderbus) that he has in the anime.
Whatever he does use as a weapon though, he uses it during the encounter with the Haniwa mob.
Edit: He apparently uses astringent juice as a weapon. XD
Death is in the house!
By the sounds of things, Death appears to be a resident within Gregory House in the manga ( his silouhette appears alongside the others in one of the manga panels) whereas in the original anime, he was someone who existed outside the hotel.
James...the dangerous child. O_o
In the manga, James appears to have gained a 'chainsaw brutality' and is almost always seen carrying a weapon around.
Such weapons would include; an axe, chainsaw, gun and knife...and yet, he still finds the time to enjoy a lollipop.
In the original anime, James was merely a naughty boy who had a rather odd sense of fun.
With that said though, the little mouse boy *did* take an interest in Catherine's syringe in the Second Guest, as well as Chef's cleaver ( and other 'cool things' in the kitchen) in the game.
Gregory the collector.
It's been mentioned that Gregory has a collecting habit in the manga, in the translation that mentions about it; it would seem that he started collecting things as a child and it just went on from there.
Dead Body appears to actually reside in the hotel instead of out in the graveyard.
By the sounds of it, he seems to enjoy sleeping.
Room 201 is inhabited!
In the manga, room 201 is inhabited by Kimiko Takahashii, although I'd probably have to play the GHS game again to see who lived in that room originally.
Lost Doll...the big head?
Yeah, this one is a little unclear to me. XD;
In the manga, instead of her head rotating when she switches personalities, Lost Doll's head gets bigger.
Her symbolism in the manga also seems to be that of someone who is self-interested ( or big-headed in the terms of thinking highly of herself)
Although it could be argued that she was actually still like that in the anime as well, especially since she was her own doll.
Angel Dog and Devil Dog, Two different entities?!
According to the translation I got, Angel Dog and Devil Dog in the manga are two completely different girls.
They do not transform into one another like they do in the anime.
And that's all I know about it so far.