(sorry about the long post - but some people may find this helpful)
From my ten years of experiences with border agents (both land and air) under student, work and now permanent visa, I have learned one thing: there is no way to predict how an individual border agent will interact with you. Although I have heard many horror stories about friends being harassed unnecessarily, personally my worst experience was when a border agent at the US-Canada land entry asked me when I was going to finish my PhD (jokingly of course, but for grad students that's a taboo question
)!
More relevant to this topic - my wife recently came back to the US after >6mo without re-entry permit - and in summary, we still have our GC and there wasn't much problem at the time of re-entry.
Details below.
A bit of background first: we got our GC last August via Consular Processing. At that time, we were living in Australia due to my wife's job (she was still employed and being paid in US by the US GC sponsoring company - she was the primary applicant). I was working for an university in Aus. After CP, we came to the US for 10 days in October and left so that we could finish our respective projects in Australia. We wanted to get a re-entry permit - and our lawyer advised us to get one even if we were coming back within a year - but due to the issue of biometrics we did not have time to get it done.
I came to the US for a conference this Feb (~4 months absence), expecting to be questioned. But the agent at LAX just said 'welcome back' - did my fingerprints and photo and that was it. I was a bit lucky possibly, as I noticed another border agent question each and every person in details about how long and even why they had been away. I went back to Australia after 12 days that time.
Both me and my wife came back to the US earlier this month. I had been away for 3 months and my wife for 8months. We were ready to be questioned and kept a few documents in hand (proof of bank accounts in US, payment of taxes, letter from wife's company stating her project overseas was over, proof that we owned a house in US etc). This time the agent at Newark airport asked us why we had been out for so long. Unfortunately, I had stupidly given my friend's address in the customs form (where I was forwarding all my mails) instead of our home address in US - which are in separate states! This raised a bit of suspicion as well. But we explained why we were away and that we were going to stay in the US from now on. It took about 10minutes of random questions - where we work, how long working etc, but he let us go without asking for a secondary inspection, or even asking to see any paperwork.
However, do note that he had a record of our entry
and exits for the last year on his computer as well as immigration history - previous H1/H4, F1s etc. In fact, the guy seemed more excited by the fact that he could actually pull up all that data and play with it, rather than deeply questioning us !
So going back to the original point - it is possible to re-enter without the REP but YMMV. Be ready all documents and answer questions confidently is my advice.
Also FYI, during both my re-entries, I was asked for fingerprints for only my right hand, while regular visitors were being asked for all ten fingers. Also, LAX had a separate line for GC holders (apart from Visitors and US Citizens); Newark had that too - but the GC line was closed and they told us to use the Citizen's line (immigration area was quite empty then).