More correctly, were found: were discovered and shown to be. See Rom 6:10; 1Co 15:15; 2Co 5:3; Phi 2:8; Phi 3:9.
Like the Gentiles, Gal 2:15. Paul assumes that this was actually the case: that, seeking to be justified in Christ, they were found to be sinners. To seek to be justified by Christ is an admission that there is no justification by works; that the seeker is unjustified, and therefore a sinner. The effort to attain justification by faith in Christ develops the consciousness of sin. It compels the seeker, whether Jew or Gentile, to put himself upon the common plane of sinners. The Jew who calls the Gentile a sinner, in seeking to be justified by faith, finds himself a sinner also. The law has failed him as a justifying agency. But Paul is careful to repudiate the false inference from this fact, stated in what immediately follows, namely, that Christ is a minister of sin.
Minister of sin
A promoter of sin by causing us to abandon the law.
God forbid (μὴ γένοιτο)
See on Rom 3:4. Not a reply merely to the question “is Christ a minister of sin?” but to the whole supposition from “if while we seek.” The question is not whether Christ is in general a minister of sin, but whether he is such in the case supposed. Paul does not assume that this false inference has been drawn by Peter or the other Jewish Christians.