Is it pretty much impossible now to be lost at sea if a cruise ship goes down, given the gps capabilities of the life rafts?

Not according to Gilligan and the Skipper.

You WOULD NOT BELIEVE some of the 'capabilities'  you can find on Panamanian registered and internationally crewed 'cruise ships' owned by Greeks, East Europeans etc.    Not least among those that have managed to obtain their safety certificates from the authorities in Puerto Rico...

Knowing where you are doesn't mean that anyone's nearby and able to rescue you; or that they know you're missing to even look

Other than the Costa Concordia in Italy, cruise ship rarely go down, and even that one was just off the coast.  I'm curious as to why you are asking.  My son's gf has never been on a cruise.  She's very nervous about flying (plane crash)  and the ship going down.  Neither are even the slightest bit likely.  

No, it's not pretty much impossible to be lost at sea. You might not make it into a life raft. If large ships, or even some small ships, go down suddenly, some or even most people never get out of the lower decks to get to a life raft. The GPS could malfunction. The life raft could get damaged, or flip over, and you can't get back in. They might not find you before you died of hypothermia. People are lost at sea almost every day of the year, technology or no technology.

Granted, technology of various kinds has certainly meant that fewer ships get in trouble, and that when they do the chances of finding survivors are better than they were even 50 years ago, and cruise ships have a pretty good safety record, but things can always go wrong.