OK, I get it now... you're right, it's a gdl issue, not a glib issue.
The GRelation API has been deprecated since glib 2.26, so I guess the G_DISABLE_DEPRECATED definition is causing the function g_relation_* declarations to disappear so the missing functions are assumed to return an integer. According to the Debian notes highlighted by ~suv in comment #6, and #9, we can summarise that the problem will occur with any 64-bit platform with glib >= 2.26.
Easy solution: Get rid of G_DISABLE_DEPRECATED to re-enable building on all affected platforms.
Proper solution: Replace use of the GRelation API in GDL and forward the changes upstream.