So for me, the idea is that you are not this child's parent and have zero responsibility for him. Your SO should be the primary parent at all times. Any help you give is voluntary and not forced, through guilt or anything else. 
 

I never was a parent to my SS. I didn't cook for him or clean up after him (my DH is the cook in our house anyway). I occasionally transported him, helped with homework and watched him, but those occasions were rare and I chose to do it to help DH. 
 

I did not discipline him. I did let DH know if SS did something wrong so HE could discipline. Essentially, I was like a fun aunt, and not a parent. I was lucky though, my DH didn't want me to parent his son, and he was a good parent with boundaries and limits. 

It's up to you for the extent you want to disengage.  One you are not the parent, you do no parent things, you are not the maid so you do no maid thing ect, No cooking, cleaning up, no babysitter for GS.  It's all up to your SO.  None of your money goes to GS in any way shape or form.  
No transportation for GS.  He GS is like a ship in the night. You say hello, good by. And talk to your GF.  Unfortunately this relationship is not going to work unless SO is ok with this. 

Unfortunately, as with most parenting issues, there is no set criteria for effective disengagement.

Some take the all in cold turkey approach. Others take the intermittent or phased approach on a select number of issues.  And anywhere in between.

I personally am not a fan of disengagement. I am a zero tolerance, all in,  total confrontation SParent.  If behavior and performance were not acceptable, I confronted it.  I did  not get into the "like or not to like" my Skid head space.  It was all about his behavioral and performance choices. The three of us have always been very close.

Now for the SpermClan, I detested then whole sale and still do.  When they stepped out of line, my DW and I brought pain and destruction down on them like lightening from above.

My SS turned out to be an outstanding man of character, honor, and performance in his life. Unlike his three younger also out of wedlock Spermidiot spawned half sibs by two other baby mamas.  One is barely keeping her nose above water, the next is in prison for felony burglary while carrying a firearm, and the youngest is not far behind.

My DW and I never accepted failure as an option for our kid (my former SS who asked me to adopt him when he was 22).

If either his mom or I had disengaged, it is not unlikely that our son would have gone down the same path followed by the shallow ands polluted end of his gene pool.

Where are the GSkid's parents btw?

Disengaging is very hard to do when their actions impact you on a regular (if not daily basis). I have two skids that are now teenagers and I've been actively involved in their lives for 14 years now. I love SS but have never been able to bond with SD and in order to preserve my mental health, have started partially disengaging from her. 

I don't feel that full disengagement is possible, it's too big a change after so many years of high/active involvement. I now get SD to ask DH for permission to go anywhere and DH does the majority of driving her around to social functions etc. The biggest change however has been in my attitude. I've stopped stressing about the outcomes for SD. She's doing very poorly at school, not my problem. She can't keep friends for very long, not my problem. She clearly has a range of undiagnosed conditions, not my problem. Her life, her choice and she can deal with the consequences of her decisions. 

This is where I think you need to get to - ie accepting that how SGS turns out is not your responsiblity and not something you should take on your shoulders and stress about. Counselling will help.