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Welcome to the Bunnings Workshop community @Jas1983. We look forward to reading more about your projects and plans for around the house and garden.
Although it's a wider gap than member @Rebbler231 in this previous discussion - How to fill gap between paving and fence? - I think many members would give you the same advice, to fill with pebbles. Some weed mat first would also be a good idea to minimise weeds growing.
There's obviously not a lot of growing space if you'd like to plant. But you could consider a project like Training climbers to screen a fence and choose a climber like Star Jasmine. There are other options in our guide How to choose a screening plant.
Let me tag some other helpful members for their thoughts: @Dave-1, @TedBear, @Jewelleryrescue.
Jason
Good Morning @Jas1983
Id like to offer a warm "Howdy" and welcome to the Bunnings community pages 
Its a pretty mice place to wander through
Because the distance from your path to the fenceline is a little wider (I think its a good thing) Maybe install some Grill Drain (thats just the splash page of a range of drains and different sizes) Along the straight edge of your path (weed mat first across the width) and White stones on top of the mat, Id really just go for stones on top as the deeper the stones the less chance for weeds to come up.
For extra longativity, I would run a garden edge along the fence base as well Saxon 150mm x 30m Black Garden Edge (that is 30m tho )
So it would be fence, garden edging, weedmat, stones, grill drain, concrete.
Dave
Do you have enough room for a long narrow raised garden bed?
