major sins and lowering the gaze whitout lust and desire

If i commit a major sin accidently do i have to repent? For example if i show my awrah when taking off my shirt am i sinful or what if i did a major sin that i did not know was a sin? If i backbite someone out of forgetfulness do i have to repent? What if someone i backbited said that it is okay if i backbite him out of forgetfulness do i still have to then repent? And is it haram to not lower my gaze if i donr look out of lust and desire? Thanks