They are completely different compounds and, in my experience, are not interchangeable in gumpaste recipes.
As was already stated they are completely different ingredients.
GumTex is a Wilton product that produces a 'paste' more like pastilige(sp?) then gumpaste. It lacks the stretchability that Tylose will give you
Tylose is closer to CMC which was originally used to make gp.
Gum tex also has an odor to it, and tylose doesn't.
As far as price, Gum tex is about $8 for a 6 oz can. I pay $4 for a 55 gram jar of tylose. 55 grams is almost 2 ounces.
Bottom line, gum tex is like a Ford and tylose is like a Lexus...they're both going to get you there but the tylose is a lot more pleasant to use 
Tylose comes from the cake decorating store, or online. I wish they would carry it at the craft stores.
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