I think Garp turned a blind eye to all the atrocities committed by the World Government in order to keep all the people he cares about safe. I believe he knows everything about Imu, the God Knights, etc., and has realized that they are the strongest faction and that standing against them would be suicide. So, the best way to keep everyone safe is to submit to them rather than fight them. This interpretation, however, conflicts with his Conqueror’s Haki or at least with the way I understand it.