It really focussed on the battle, giving only the essential contextual information about the wider campaign.
I found the early engagements between the French and the Prussians inspiring and think Gilly might work as a small scenario for Napoleon's Battles, although perhaps more suited to rules at the battalion level.
What I now have to accommodate in my scenario is the absence of the 14th Division which was engaged in skirmishing with the III Korps, the arrival of the 5th Cavalry Division as a reinforcement (17:30), that it was only a brigade of the 1st Cavalry Division that arrived, and of course the impact the appearance of the 1st Corps had on the French left wing (18:00-18:30). It would also be nice to have the news from Quatre Bras impact the Prussians (which I think is going to be needed as a game balance. It was received around 18:00 and so the Prussian new the "dark masses" were French reinforcements). Then there is the thunderstorm (around 20:00).
There was no mention of the covering attack by the 9th Brigade at the end of the battle, although certainly the work of the III Korps in assisting the safe withdrawal of the Prussian army is mentioned. I'm guessing the Osprey format with it's fixed number of pages meant somethings had to be left out, or maybe the other sources that I have read thought it was more significant than it actually was.