baos = new ByteArrayOutputStream()
out = new PrintWriter(new DeflaterOutputStream(baos))
out.println("hello, world!")
fileWrite("/path/to/file", baos.toByteArray())

I haven't tested that, but it should be close.

I don't suppose you wrote the reverse of this function by any chance? I've been struggling with Output Streams etc, but not getting anywhere… (I'm actually trying to deflate, not gzip but it's pretty much the same process I believe)

I wonder if the exception was do to the wholesale conversion from the ByteArrayOutputStream buffer to a String. I haven't cracked 10MB yet, so I can't say if mine will have the same problem, but I'm doing that conversion a piece at a time (via the Reader/StringBuilder arrangement). More simply, I'm reading textual characters a line at a time, rather than bytes 1024 at a time, into my output "thing". I'll definitely post back if/when I cross that threshold though.

I wrote a similar function a couple years ago:
http://blog.pengoworks.com/blogger/index.cfm?action=blog:501

You're using a StringBuffer and I used a ByteArrayOutputStream, but I ran into some Buffer Overflow errors if the GZIP'ed content was too large.

In my case I was reading in an XML packet that had been GZIP'ed and the XML files could get pretty large.

Just something you might want to test for…