gouden.

From Middle Dutch gouden, goudin; essentially goud + -en. This form, a Middle Dutch formation, is much more recent in origin than its synonym gulden. The reason for this re-formation of gulden as gouden lies in the gradual divergence of the pronunciation of goud (from Germanic *gulþą) from that of gulden (from Germanic *gulþīnaz). While the original Germanic forms are evidently related, this relationship is much less clearly visible in the forms goud / gulden which existed from Middle Dutch onwards, allowing for the adoption of a new adjective based on goud (which more closely matched its pronunciation than gulden did), namely gouden. Finally, the reason why goud developed its pronunciation the way it did and gulden did not, is that Germanic *gulþą underwent a-mutation to golt (the Old Dutch form), triggering the sound change from -olt / -old to -oud regularly observed in Middle Dutch (cf. e.g. oud, koud). The etymon of gulden however, Germanic *gulþīnaz, did not undergo a-mutation as its suffix prevented this (it instead underwent i-mutation). The resulting Middle Dutch form, gulden, therefore remained unaffected by the above-mentioned sound change.