Last time I checked, using GCJ was constraining yourself to java 1.1.x, since even the java 1.2.x work wasn't complete. I'm assuming by now they've finished that, but IIRC they haven't got 1.4 ready yet?
As I understand it, GCJ relies heavily on the ClassPath project. This makes it sort of a weird combination of 1.1 and 1.4 libraries. i.e. You can have ByteBuffers, but no BufferedImage. Thus a developer would need to cover the missing pieces with some sort of application specific API. In this case, LWJGL actually makes a lot of sense.
So...works in theory, but in practice it's not doable? If all you want is to write java 1.1 code, well...a 1.1 JVM isn't too hard to write. But what we actually want/need is the level of arch-specific and clevel runtime optimization in a Sun or IBM JVM, and that's a whole different kettle of fish.
I think it's quite doable. The only issue is that you have to define the constraints up front. i.e. These libraries will work, these won't, and this is how these will fit.
On a side note, I wouldn't listen to a single word J Marner had to say, based on the atrocious quality of his previous work, until/unless he owned up to and corrected his previous mistakes.
It was just the first reference I came up with. Don't shoot the messenger. :-)