So sorry that you find yourself in this sad situation. I think you have to take a deep breath, grit your teeth and try and accept that you are in this for the long haul. These sort of rifts need careful thought, just chipping away bit by bit, keeping the door open whilst finding the strength to recognise that you must get on with your own life - sad though you are, you cannot and must not put everything on hold. You only have one life and this is a stumbling block along the way.
It is clearly a very delicate situation. Your DD's BF does not sound as though he is the sort of person who would take kindly to criticism, however carefully worded. I am sure that we can all see the possibility that this BF has gone in with too heavy a hand, trying to establish his rules for your GS rather than being sensitive to everyone's needs.
Re-establishing relationships is not going to be easy, as once people dig their heels in it is hard to dislodge them. It may be that your communications with them need to apologise for expressing your views about his handling of your GS - I know this is hard for you, but it is likely to be the only thing that might change their minds over time. You are probably right about his approach lacking sensitivity, but the rights and wrongs are almost irrelevant in trying to mend this rift. The priority is to create this mend so that you and your GS can pick up the pieces of your relationship and you can be there for him to deal with this minefield that he finds himself in.
Given that on one occasion they asked you to care for DGS while they went on holiday and without notice, it may be that their need to have someone to look after him while they pursue their new life might be a glimmer of hope. They may get fed up with having no built in baby sitter and this may draw them back to making contact with you. Eat a bit of humble pie, even though you feel you are in the right, as this is the best way to achieve the end goal.
This must all be so painful for you and I do feel for you.