Found these batteries on Ebay and just wondered what the weight difference was going to be  - I expected the Gel battery to be heavier than the standard Yuasa? battery for my 900ss but was quite surprised when I emailed the guy.....
  "YB16 is 3.85kg, 12v12b4 is 2.4kg" 
 Here is a list of his Ebay items:  
Westco Batteries  I've had some weird dealings with a different Gel battery in the past but its turns out that might actually be the fault of my bike 
  Dunno anything about these otherwise. 
 Good luck - Frank