In order of quantity, Russia, The Netherlands and Norway and rather less from other countries.
Meters connect the two systems.
The main issue is metering of the gas flow for which we use the ISO standards. Take the Russian supply. A pipeline 5000 kilometres long from Siberia crosses the German-Czech border at Waidhaus. The flow and energy content have to be measured scientifically. A small resident team of engineers from Gazprom the supplier and the consortia from Europe permanently monitors the flow, reconciles the logs and signs the schedule.