I have taken several prescriptions from a variety of companies since being gluten free. I have called on every one before I fill them. Every time I get "but we don't promise/guarantee it is gluten free blah blah blah statement." Having previously given similar statements myself working in a call center, we called these CYB statements. Cover Your Butt. Like Peter says, the legal implications of making an absolute guarantee is insane, the best you will get is a general, yes, the drug is in fact gluten free but CYB.
Thanks for the CYB info--might take omeprazole again--after I get some gluten free time under my belt. I'm only recently diagnosed & glutin free 4 days now. Trying to take it "One day at a Time'--but REALLY hard at this point.