I work with sealed lead-acid gel-cell batteries every day, in my job as a fire alarm system service technician.
  I've seen of them many mounted in every possible way (and sitting for years!) with practically no spillage at all.
  So, yes, on it's side is absolutely fine; as advertised by the manfacturers.

Mike B.
  ----- Original Message ----- 
  From: Paul Gordon 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Friday, November 20, 2009 5:33 PM
  Subject: Gel Cell's

  Anyone ever had an "Negative Experience" mounting these on their sides. They claim not to spill, But what about leakage. I think I can get a larger battery this way but not sure about installing it on it's side.Any insight would be helpful.