"Gap" might not be the best since the verb is marked with a particle, but the marker doesn't agree with anything.
mirrō ukru nīkeaþ ševa saìes
mirrō u- kru nīke -aþ ševa saì -es
cat REL- kill mouse -OBJ go river -LOC
The cat that killed the mouse walked to/from/along the river.
There's a word forming relative clause "meg" and the counterpart of "meg" before vowels is "megin", "meg" and "megin" are differentiated by phonological conditions, not grammatical conditions.
Relative clauses are marked with a relativizer that does not mark for case. The WALS article seems to consider cases like these as examples of the "gap" strategy.
Gap relativizer, r(i)-= is used for restrictive relative clauses.
For non-restrictive relative clauses the pronoun-retention strategy (and no relativizer) is used.
