1)
What is the meaning of "Tarbus Anashim Chata'im"
1.
Targum Onkelos and Targum Yonasan: It means 'Disciples of sinning men'.
2)
Why did Moshe refer to the B'nei Gad and the B'nei Reuven as "Tarbus Anasim Chata'im"?
1.
Oznayim la'Torah (citing the Yalkut Shimoni): Indeed, he should not have done so, and because he did, his grandson worshipped the image of Michah.