my garage of this type, at my old house, had wood vertically, then a horizontal piece that went above the corrugated material, covering that gap.
I replaced it while I lived there as it was rotten, and in the end I replaced in the same fashion. However this was because I couldn't undo without destroying, the hooks holding the roof down to the steel frame. If I had been able to do that, I would have spaced the sheets out a bit more (they were overlapped a lot in most places) and then created a situation where the vertical wood or board, could have been tucked under the first corrugation at each side, and do away for the need for a gap filler or horizontal piece.
If you have bit of time, a grinder and replacement hooks, or buy some extra roof sheet and extend the roof each side, this might be a better and more long lasting solution than trying to fill the gap.