I booted windows and plugged an USB stick in. Then I called the ASUS installer and installed express gate to the USB. The wizard asked to which partition express gate shall be installed.
I chose booting from the stick (in BIOS) but that didn't work. Express gate's error message occurred while booting "... no express gate installed ..." or similar.
I didn't have a look at the files that were installed to the stick. But I think it was about 300 MB of space used.