Everything of value of this subsidiary has (pipelines, stored gas, etc), is physically in Germany and already subject to German law. The stored gas wasn’t going anywhere without the German regulator’s permission and the regulator can always force Gazprom Germania to sell German stored gas to Germans.
Nor does seizing this magically solve the problem of lack of gas. The Germans either abide by Russian rules for gas, or pipelines coming in from Russia remain empty.
This was a boneheaded move. The Russians can now seize a comparable German asset in Russia to recompense Gazprom; say a VW plant, a real asset that can continue to produce real things while the Germans are left with old laptops, empty pipes, a salt cavern and unheated offices in Berlin.