About a year ago I looked into stonework repair techniques. There’s a lot of info on YouTube mainly aimed at repairing marble floors and stone worktops. There are various products and techniques for patching and invisible mending. e.g. https://youtu.be/8wDn3DA1WOM?si=KtE9E8gNXLqKgVYh
That gap - or chasm - is pretty big though. My thought if I was doing it myself would be to try and get an offcut that matches well and first off fill the gap with that once the worktop is level, solid and properly supported. You might find a suitable offcut in a skip like the one in Walthamstow - see below. You could also ask a place that does worktops to cut a sliver from a granite offcut. You’d then be able to work on making the cracks invisible.
The quick and easy DIY alternative might be to get a kitchen worktop joint in black or granite effect and stick it in with loadsa glue like they did before. It wouldn’t be good, but it would still be a lot better.
But ask @lewisss or @joeatkin2 they’d know from experience, which is much better.