Gabble's implementation of (standard, XEP-compliant) Jingle is rather old, and needs updating. Vaguely assigned to Senko.
Fixed in release 0.7.16 with the merge of the big jingle branch.
This brings us up to speed with v0.32 of the XEP-0166 draft,
while we retain backwards compatibility with our previous
selves (circa v0.15 of the draft).