simple as :

gst-launch-0.10 pulsesrc device=alsa_output.pci-0000_00_1b.0.analog-stereo.monitor ! audioconvert ! lame bitrate=256 mode=stereo ! filesink location=input.mp3

The device name (alsa_output.pci-0000_00_1b.0.analog-stereo for me) may vary. You can use the command : pactl list to list all pulseaudio connections, grab the name for “Destination #0”. Note that you need to add a .monitor extension when you call it with gst-launch