JEMz, IDK if you're still reading here or not, but if you are my heart goes out to you! Even though you will soon see your beloved GS, I get that you're feeling insecure about your whole relationship w/ DD - and b/c of that w/ GS - and I totally understand that.
I agree that it is completely reasonable for you to have some toys at your home for GS to enjoy when he's there, and at your mum's for him to play w/ at her house. Also, I feel DD sends mixed messages when she asks you not to bring anymore toys to her house and then complains when you don't buy a Lego set for her home.
But it definitely seems as if this is about more than the Legos or any other toys, and that your relationship w/ DD is a checkered one. It may very well be, as PPs (previous posters) have suggested, that she is jealous of your relationship w/ GS.
If you don't mind, I think there are a few mistakes that you may be making, too - well-intentioned, no doubt, but, unfortunately, exacerbating the situation. Mostly, they fall under the heading of not considering/respecting DD's wishes for her child or her home. For example, as a PP pointed out, you promised GS a toy for his house w/o checking w/ his mum/DD first. And I'm sorry to add, even though you knew DD said she didn't want you to send any more toys. When DD asked you to ask about seeing GS, yourself, you turned around and had your mum message her instead. I understand why, I really do. But can you see that she may have seen this as another disrespect of her wishes? In the future, I strongly recommend that you take her wishes more seriously. You won't always be able to accommodate them, of course. But it seems to me it would be worth trying to do so a little more.
IMO, there is hope here. Look how it worked when you respected her wish that you contact her partner about visiting w/ GS! It may not have come about as quickly and easily as you would like, but you are getting to see him. That tells me you don't have to "forget they exist," you just need to respect DD's wishes, if possible, and it, apparently, goes a long way.
"I thought I needed time away from my daughter so I decided to not contact him as I usually did every week"
I get why you felt you needed time away from DD. And I realize that to talk w/ GS, you would first have to talk w/ her. But can you see your decision not to contact HIM as usual may have seemed to her as if you were punishing HIM for the conflict between you and her? And do you realize that you are the one who initiated this semi-estrangement, not DD? Some posters are reacting as if she CO (cut off) you, but she didn't - you CO her.
And if you think about it, I'm sure you'll realize that it's difficult to get to see the GC when you (general GP) CO the mum. Fortunately, in this case, you can arrange to see him through the dad. Please just enjoy the visit and don't brush off the chance for future visits if it's there. Keep working through the dad if you don't feel you can approach DD. Please also accept that the visits may not be as frequent as they were in the past and that you need to fill your life w/ other things (friends, hobbies, volunteering, your work, whatever keeps your mind off your strained relationship w/ your DD).