Ideally gradual retreat wants to start right from word go, a way of dealing with your newborn as they grow.
So when my DD was a tiny newborn baby she would be fed, winded (as a means to ever so slightly rouse her) and placed in side-car cot. I would lie on the bed and cuddle right into the cot, often BF whilst in there if needed. With her lying in the cot.
She grew past the fourth trimester, newborn phase at 12 weeks and I found I didn't need to cuddle her in the cot, I could feed and then just lie right next to her, head to head, and place my hands on her to sooth.
She grew past 4 months and I was able to put the cot side back on. Still lie next to her to sooth, sometimes placing my hand on her for reassurance, sometimes just being near her was all the reassurance she needed.
She grew past 6 months and I could lie at the other side of the bed, as long as she could see me she would settle.
She grew past 8 months and we lowered the cot side and moved her into her own room. She needed me next to the bed as she went to sleep, sometimes with a reassuring hand on her but mostly not.
She grew past 9 months and I was able to sit a few meters away from the bed as she goes to sleep.
She is now 10 months and I can stay in the doorway as she goes to sleep. Usually asleep (with dummy and blankie) within 5 minutes, 10 at the most.
So to answer your question OP = Gradual withdrawal, like most sleep training methods, are generally not something that is easy if you just decide one day that "right, we are going to do it like this now".
If it is a 'from birth' ethos though - then it works without any tears or distress.