What blessing is a said on a food that contains gluten?[1]

Foods which are made from non-five grain ingredients, but contains pure gluten which derives from wheat or another one of the five grains, does not become Mezonos due to the gluten.[2] Thus, one who adds gluten to legume bread, the bread would nevertheless remain Shehakol.

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[1] See Vezos Habracha Birurim 24-2  and that so ruled to him Rav Moshe Shternbuch, Rav Pinchus Sheinberg, Badatz Machzikei Hadaas, and so was the leaning opinion of Rav SZ”A; Piskeiy Teshuvos 208:2

Other opinions: Some Poskim question that perhaps gluten should be viewed similar to flour, and hence could make a food become Mezonos if it is added as an ingredient of taste.  [See Vezos Habracha ibid]

[2] The reason: Although flour of the five grains is always considered the main ingredient of a food, and hence the blessing of the food should be Mezonos even if it is a minority ingredient [Admur Seder 3:2; Luach 4:2; Admur 168:11; 212:1; Michaber 208:2; M”A 212:1; Rav Kahana Brachos 36b and Rava Brachos 37b], nevertheless, pure gluten is not considered similar to the flour of the grain, and hence does not carry over its Mezonos status. [Vezos Habracha ibid] This is in addition to the fact that it is mainly added for the purpose of making the food texture more elastic, and not as an essential ingredient of the foods taste or satiation ability. [See Admur Seder 3:3; Luach 4:3; Admur 169:9; Michaber 208:2; 204:12; M”A 204:25; Taz 204:16; Brachos 39a]