I have just changed bikes from a GS with Touratech Zega panniers with bottle and fuel holders to a GSA with GSA panniers.
I have tried to take one of the holders from the old panniers and fit on the new ones and there is a small problem but it does not seem to be impossible.
The problem is the very large bevel on all corners of the GSA panniers. Logically the bottle holders should be fitted with the bottom edge aligned with the start of the bevel. This results in the top of the holder interfering with the pannier lock. If the holders are fitted, as with the Zegas, with the bottom edge aligned with the bottom of the pannier they clear the locks OK but will look a little odd.
The solution I intend to try is to buy the quick release holders Touratech have for their new super-duper panniers. These allow the holders to be removed when not needed so although they will still have to be fitted with the bottom of the bracket aligned with the bottom of the pannier I can easily remove the holder when not needed, just leaving the little quick release brackets on the pannier.