Hi Grace,
Yes, this is a bit tricky,
Certainly there is a tear of the medial gastrocnemius aponeurosis,
as for the FGA, from the images I think the tear is likely too proximal to be an FGA failure and as you pointy out there is no obvious match head sign.
I find short axis imaging helpful to sort these out as when you come up the medial aspect of the achilles you will see the puckered up match head in short axis. In your case I can’t se such a match head and hence I am suggesting that it is more likely to represent an aponeurosis based injury rather than a FGA.
It is also interesting that the aponeurotic space looks thickened, especially on the first image which makes me think there has been a similar prior injury.
It looks like a bit of chronic granulation change from a prior medial calf tear.
Steve