I am wondering if it would be possible to dye black gorilla glue and use it to fill cracks in wood instead of the well known black epoxy method?
No. Polyurethane glues foam up as they cure, and the foam isn't particularly strong. If you try to use it to fill cracks, you won't get the hard, solid surface that you get with epoxy, but instead a crappy, brittle foam.
Because finding right epoxy and dealing with it is much harder than glue.
At least here in the US, suitable two-part epoxies are readily available in any hardware store or home center. I don't know if that's the case in the UK, but Amazon and other online suppliers also offer many options. Aside from the 10 seconds that mixing the parts requires, I don't think there's any real difference in effort required to use epoxy compared to glue.
can gorilla glue sanding give decent results for small surfaces? like continuous matte surface?
Try it. Just fill a crack with plain, un-dyed Gorilla Glue as a simple test. I don't think you'll like the results. As I said above, you're not going to get a solid surface because the glue will foam up as it reacts with moisture in the air.
Gorilla Glue is great stuff -- it's extremely strong, and it works well for outdoor applications where it resists moisture and UV light very well. It's just not good at filling gaps the way epoxy does.