When using the IsRelative setting to true GMS pushes the query throught Axes SelectItem. Sitecore does not allow fast query for Axes selects, e.g.:
Item.Axes.SelectItems("fast:./*");
See documentation here page 3:
http://www.iosh.co.uk/~/media/using%20sitecore%20fast%20query001.ashx
However GMS being awesome allows us to solve this another way, you can put placeholders in your query that GMS will expand. Removing the IsRelative property and using the {path} placeholder allows the same result:
[SitecoreQuery("fast:{path}//*[@@templateid = '{011EC776-D9F3-4D73-8D8D-E454417E7574}']")]
IEnumerable<ItineraryLine> Itinerary { get; set; }
The path placeholder will be expanded to the full path of the current item being mapped.