It is my understanding that Gel ink is not kiteable or easy for a forger to remove without destroying the substrate.Gel ink is a semi liquid form of ink where ball point ink is a paste.
However any pen that will accept a parker style ball point refill can be refilled with a gel refill. Parker , Itoya, Schmidt,Yafa and Private Reserve all make gel refills.
Any good pen store will have these but if you cannot find one try Montblanc & Mont Blanc fountain pens, Parker Pens, Waterman pens.
or contact me and I send you one.
Penmon aka jim