What do you recommend to fill in the gap between the concrete and the decking.
Unfortunately the company that did the contrete did not do it straight in line with the building.
@dany remove the board thats there and replace it with a wider one scribed to the suit the concrete.
Welcome to the Bunnings Workshop community @dany. It's terrific that you've joined us, and many thanks for your question about decking.
@stackhat's suggestion sounds like a good option, and using 140 x 19mm Kwila Timber Griptread Decking could work. However, I think whatever option you go with it is going to look a little peculiar. Tapering a board to suit is likely going to be one of the easier options, so I'd suggest you give that a go to see if you are happy with the results.
The only other option I can think of would be to remove that last board, form up the edge of the slab and pour an additional section to square it off. This would obviously be a significant amount of work as you'd also need to try and match the Pebblecrete. You'll likely also run into an issue down the road as due to needing to feather in this new portion, it could crack in the thinner area.
Let me mention @TedBear to see if he has any thoughts.
I tend to agree with @stackhat 's suggestion. It would be easier to shape the last piece of timber to suit the gap. Unfortunately you will also need to replace the end frame piece too, so that it fits right into the corner. The grano worker didn't do the job properly, but it's too late to fix that now.
I bit left of field here, but have you considered the opposite direction and hiring a concrete saw to make a slight cut in the concrete to square it so the gap is the same the whole way. Could be an easier solution and making it uniform could look good.
Could you take a zoomed out photo to see what the area looks like. Maybe you coud put a screening along that line that would cover that up and asthetically look good too ... but depends what the bigger space as a whole looks like.
Or maybe even putting a raised garden bed along that concrete line could look good too.
Again, all depends what the broader space looks like and we can't see from the photo.