As you mentioned, what a reader feels when they read about a gory sight - more generally, what a reader feels when they read anything evocative - is strongly influenced by what comes before. If you think in terms of horror films, the effect of the reveal is heightened by the tension leading up to it; it's why we have things like slow, lingering shots of the hero(ine) reaching towards the door handle, rather than just cutting from them deciding to search the house directly to whatever they find.
In other words, it's less about what exactly you write and more about putting your reader in a state of mind that whatever you do write (within reason, of course) will seem scary rather than silly.
In general terms, this means foreshadowing the gore, whether directly:
"As he walked towards the door, he saw a few dark red spots of blood dotting the carpet in front of it."
...or through the viewpoint character's thoughts and actions:
"He took a deep breath and reached for the door's handle, trying to push away thoughts of what might be on the other side."
...and keeping the pace fast enough to be moving towards the reveal, but slow enough to be suspenseful.
In a more granular sense, short sentences and "sharp" words can create a feeling of tension, if you go in for that sort of analytical writing.
If there's no opportunity to foreshadow the gore in terms of in-work timeline - for example, if it is a total surprise to the viewpoint character and you can't convey any apprehension beforehand - you can still create a lead-up to the reveal for the reader. A good way to do this is to describe a character's reaction before describing what they're reacting to:
"His breath caught in his throat as he tried not to retch at the sight. He wanted to look away, needed to tear his eyes away, but he could not. Add more here.
"Lying in a pool of blood in the centre of the room was..."
As I said, get your readers in the right state of mind before the gore, and they'll likely take the gore the way you intend.
In terms of not making it seem parodical or silly, I'd say the key is just to be reasonable. 846
Use a wide vocabulary, but keep it simple. The two possible mistakes that come to mind right now are being overly technical, and being melodramatic. Don't write a coroner's report; the point is to have your reader empathise with the fear or horror felt by that character, so use the words that your character is probably using in their mental narrative as they see the gore. On the other hand, avoid overly dramatic diction; again, it's just a matter of considering what sort of words your character is probably "thinking with."
Again, empathy is key, so visceral, emotive words that show the character's response to the scene can be very effective. Overall, it terms of diction, just use your common sense; you will probably be able to see if something is becoming silly rather than serious.
If the description of the scene doesn't have a character - if a disembodied narrator is describing a scene for the reader's benefit alone - it will be much harder to get the reaction you want in the reader. That said, if you must do this, I would generally recommend using the diction that your lead protagonist would use in his/her mental narrative of the scene if he were there to see it. 846