The type certificate holder TN on control surface seals is available at:

http://www.ltb-lindner.com/service-letter.html

Second from the bottom of the list on that page.

They specify a positive air seal on the controls "to prevent flutter". It's just a TN and with no AD associated with it so I believe the seals are optional rather than mandatory. I've not heard of any flutter problems with Grobs with the exception of the later "III" series 102's and that had nothing to do with seals but rather with a lack of mass balance. We own a Standard III and it now has huge hinges and big external mass balances as part of the AD that corrected that problem. That particular ship did in fact have elevator flutter once. Actually so did our Acro but that was the result of botched aerobatics which resulted in exceeding the VNE. On our two club Grobs we have sealed the surfaces in accordance with the TN in the simplest way - Tesa fabric tape on the hinge side of the control surfaces.