The gun is a metaphor for, among other things, US backed coups. Sanctions against Cuba are another example.
This is an old meme format where the foreground character shoots the background character, then blames the flaws of the background character on something unrelated to being shot.
I genuinely recommend reading the book, it won’t take you that long.
Key points I got are:
Summary of the US policy toward Russia post USSR up to present
There is a history of NATO moving east, and also a history of US weapons testing near the border and backing out of nuclear and arms treaties.
Preliminary integration of Ukraine military and economy prior to any admittance into NATO, effectively making them an arm of NATO without formal admission
A bunch of other history which contextualizes things. Seriously good extra context if you are not familiar with the history.
Ultimately, the US and NATO are far more at fault for the tensions that led to the current crisis.