No, I don't know whether there are plans to support properties, but if GNUstep plans to stay viable (even if only in the limited degree to which it is now) this should be a priority. Unless GNUstep decides to adopt Objective-C 2.0 features, the gap between it and Apple's implementation will make it increasingly difficult to write good cross-platform code. (It's already borderline pointless for most developers.)
Although I usually hate answers that essentially say "don't do that, it's a bad idea", I have to agree with @Jonathan on this one, especially from the standpoint of practicality. Although the code may compile cross-platform, if users have to install a runtime just to use your app, the probability that someone will use your app is vastly diminished.
This SO answer sums it up nicely. I suggest reading it and draw your own conclusion.
It's also worth considering that the "objective-c" tag has over 3280 questions, while "gnustep" has 9. I'm not saying that question volume is a metric of quality, but it is a parallel with activity and interest, and quite possibly a low number of people with expertise in GNUstep on this site. Thus, you're less likely to get good help if you choose to go down the path you're considering.
By the way, the mentality of avoiding new features in the name of compatibility is a "least common denominator" behavior that, in the long run, will make your code less elegant and less "featureful". It's similar to only coding with APIs available on 10.2 or 10.3 — any recent OS X or iPhone developer will tell you they'd rather take advantage of the cool new features, and not be hampered by past limitations. These days, new apps almost always require 10.5 — support for older versions is more characteristic of established software that is being backward compatible, and many apps even drop old OS's over time.
If you're considering selling an application, using only GNUstep-compatible APIs will severely limit your market, and even limit your app in fundamental ways, including the level of polish and functionality you can hope to achieve. Even if the app won't be commercial, there is generally merit to using the language and frameworks that are the best fit for a given platform. If you're really looking for cross-platform support, Java is likely to get you closer with less heartburn. (Java is definitely not my favorite language, and it's not Cocoa, but it does many things well.) Although there is still the same problem of language version across clients and platforms, at least it's designed to be cross-platform, and all consumer platforms have solid Java support.