How would a grandmaster handle a opening he is not familiar with?

They say a grandmaster doesn't "care" about the openings thats much, at least , not as much as I would do as a 1100-ish player, but rather about the overall tactical devolpement( or some like that).

But I wonder, how would a grandmaster handle an opening he is not familiair with? I mean, it's impossible to memorise every opening that's out there, right? How do they look at the game from that perpsective?

There are 3 basics for every opening. If a grandmaster would not know an opening (very unlikely in a normal opening) he would fall back to these principles. 

1) Development 

2) Castling 

3) Connection of the rooks

Ofcourse grandmasters have a much better feel for positions and dynamics and a much better tactical eye and they are able to beat most weird setups behind the board. 

I remember the comment of Jan Gustafsson on some strange gambit on his e4/e5 dvd stating .. yeah it is weird and not very good. I never looked in to it but i am pretty sure i can refute it behind the board. 

A grandmaster would probably have all bases covered with regards to openings.Even with wierd setups and "junk" moves like 1.g4 or 1.b4 they have a general idea on what to do.If they don't know the refutation they will try to stir it to a familiar position.And work things from there.

They usually have seen many, many games during their chess lives 

So chances are they already have some idea how to handle every common opening, and can build up by applying their knowledge and adequate thought-process in any specific opening position.

What about players who just do batshit crazy stuff in the opening, like sacrificing their knights to draw your king from safety, exposing you to attack and removing the option for castling?

It seems very common especially in nonrated games.

In a GM game the GM would just be like "Thanks for the gift!" after some random Nxf2/f7 sac. This is excluding the 1 or 2 Topalov games against Kramnik (I think?) where this seemingly random sac happened I think. But it was well prepared/calculated so not really random at all.

trade off pieces and pray

Yeah, but if you give a Knight without compensation to a GM, he will probably win...

Yes, and Topalov is not your usual 1100-rated guy either...

im pretty sure that is the case

They say a grandmaster doesn't "care" about the openings thats much, at least , not as much as I would do as a 1100-ish player

Ummm, who says this? ;)

It's not remotely true.  While you could say that many GMs might eventually "discard" deeper knowledge of every variation out there (because at the highest levels, a GM does not really need to know how to play against the Danish Gambit move by move, for example, they can beat it with general knowledge of how to equalize), they nevertheless usually know enough about these openings to be very comfortable refuting the less strong openings based on principles of the opening, if not knowing specific moves.

You can rest assured, though, that any GM knows a ton more about playing the most popular openings than any 1100 player.  At a bare minimum, they would have to know at least 4 openings inside out and upside down...how to play against e4 e5, e4 c5, d4 d5, and d4 "everything-but-d5".  And they would probably know pretty much every variation of their opening arsenal out to 15-25 moves...maybe into the 30s for the most cutting edge lines that are being played currently.

When 2 GMs play a Sicilian line, other than the choices of variations, the real "game" and decision-making doesn't even start until they are 15-30 moves in and reach the end of book lines (or spring a carefully prepared novelty, perhaps)...

and gms i believe even say openings get more important at higher level.

well besides the strategies you need to know for opening is really also important for middle game and several other aspects. I dont think you really can play chess without really having a clue about every aspect of the game.

i mean the opening is the foundation for the entire game.

This just isn't true. Just a couple of weeks ago Caruana said nearly all his time is spent on opening preparation.

Openings are based off of logical decision making and proper positional assessment. Grandmasters are good at these sort of things.

but some moves have consequences so deep even GMs have to prepare or they probably will lose on time.

I won't argue that, but it's a rare case that not only does the GM not know the theory of the opening he chose, but is also not familiar with the middle game position that arises from it.

Do you actually think a GM would play into a "deep consequence" type of opening without knowing anything at all about it? If they did, they'll lose (against an opponent who does know). GMs do lose; it happens.

Oh I'm sure that would not be necessary for them haha

If a GM gives me a Rook, Knight and 3 pawns without compensation, he'll still probably win.

Erm, 2 & 3 are inter-related and certainly do not warrant 2 of 3 possible basics, when you left out something glaringly obvious: central control

that was in the cochrane gambit, which IS a well known opening, it definately wouldnt have come as THAT much of a surprise.