I've always thought of a garden gate as being a symbolism for innocence, and this could be referring to Bono looking at his life up to that point (Bono has said that 'Out of Control' was written on his 17 birthday when he suddenly realized that he was moving from childhood to adulthood.). Bono writes of trying to fight the very fates that sometimes befall us, as such the case of losing of his mother at 14. So Bono refers to the garden gate of childhood innocence as painful in his case, and that he realizes that this is his lot in life, and that this feeling of 'out of control' stems from not being able to stop that which is painful, that which would change the course of his early life forever. The 'out of control' that Bono could be referring to could also be the 'troubles' that many children experienced in the early 70's in Ireland, particularly Belfast, and how in an instant of violence, their childhood was lost.
Chris