I used gentamicin eye drops before I knew I was pregnant. I am 6 weeks now and have stopped. What is the chance there is major damage done to my baby?

General/pregnancy. In general, any drop/cream/ointment/etc. that you apply to your body surfaces cannot make its way into the blood at a level it could reach and harm a fetus. The only exception to that rule are those specialty patches that have a special chemical allowing meds to move through the skin.This drop specifically would not be a problem.
Don't worry. The chance is minuscule. I wouldn't worry.
Minimal. Gentamicin eye drops pose minimal risk to your baby since very little is absorbed systemically.