Hi there,
My 2.5 yo DS also has GDD. He has DiGeorge Syndrome, also known by the catchy title 22q11.2 deletion syndrome.
He has had issues with low muscle tone and didn't sit up until he was about 13 months. He started walking at 21 months and runs everywhere now 
He is non-verbal too and his only sign is 'more'. We have tried so hard to get him into signing but he just hasn't picked it up. I think this is because he doesn't instinctively 'mirror' and so we have to teach him using 'hand over hand' instead.
He is in mainstream nursery and has been since 11 months, but doesn't have a 1-1 yet. We had Portage for about 9 months and then got discharged as he had made a lot of progress. He is still delayed over all, especially with communication, but he didn't meet their criteria any more. I am still absolutely gutted about losing the support TBH.
The developmental paediatrician says we are 'not going down the autism road' just yet. Which I think means that, while he does display some ASD traits, there is not enough to make that diagnosis just yet. My gut feeling is that we will end up with a diagnosis of LDs rather than ASD, but who knows?
We have found a private speech and language therapist and hoping to move forwards with PECS or visuals as I think we really need to find a functional method of communication for him. The longer we leave it the more frustrated he will become.
It is hard work, and difficult to explain to people, especially when they try to brush it off by saying 'he'll get there in the end'. They don't understand that he is missing out now and that actually he might never catch up 
He is our awesome little dude, and is pretty happy most of the time. Crap at sleeping but good at making us laugh 💕