KYB doesn't make Gas Adjusts for the front shocks on the Vanagon only the
GR-2.  I have installed several sets of these and the vans rode fine.
You problem sounds like the shocks are still loose (the nut of the top is not
tightened all the way or the bolt on the bottom isn't tight), maybe your
spacers that go around the bottom bolt were not installed again when they
reinstalled the shock.  Or the shock/shocks are defective.  I usually pump
these shocks through several times before installing them (KYB's manual says
to do this) and this would show up a defective shock like you are describing
pretty quick.
Hope this helps.

Ken Wilford
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242