If it was, in fact, Gorilla Glue or something like that, steaming will soften the wood but won't (at achievable temperatures) do much to soften the glue.
If it is an older job, the alternative to hide glue that was used for "high strength" assembly - what held most wooden airplanes together - was chicken blood. It sets up very hard, and is almost impervious to both heat and water.
If steaming is not working, the next best bet would be carefull use of a sander or plane to just cut down to the glue line. Since you're replacing the veneer, you're not worried about taking it of in little bits. Note that sanding wet wood is rather inefficient, so you may want to let the effects of the steaming "breathe" for a day before changing to that method.