The so-called tau gallicum was a character used in Gaulish, written Đ, ð or even a Θ. Its name comes from the only commentary on it that we have, by Vergil (Appendix Vergiliana, Catalepton II, 4).
Every study I have read on Gaulish states that it was pronounced as the affricate [t͡s] or the reversed [s͡t]1, but there are occurrences of geminated tau gallicum, in the Chamalières' lead and in this epitaph.
I am really not comfortable with a [t͡st͡s]`in snIeððic or aððedillI, so I wondered if there are others hypothesis for the pronunciation of this character. Maybe [Θ], since Θ and Greek Θ are obviously close, at least in writing.
1. See, for example, Lambert's la langue gauloise "The affricate /st/ in snies-ti-c is written striked d, as usual in Gaulish."