Open the Index (bottom of your Bastion screen - banner icon).

You will see the factions are groupped by four (except for the poor lonely Dwarves). The Gaellen pact consists of Ogryn Tribes, Lizardmen, Skinwalkers and Orcs. 

Champions from Ogryn Tribes, Lizardmen, Skinwalkers, and Orcs.

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Cool, thanks for showing that. Looked at that screen hundreds of times and never paid enough attention to pick that up ;)