The shares were priced at 50.5p each. The proceeds will allow Gusar to finance the purchase of two POS-GRIP jack-up exploration wellhead systems from Plexus.
Plexus said the wellhead systems and associated equipment will be shipped to Gusar once the buyback is completed.
Shareholders unanimously voted in favour of the proposal at a general meeting on Friday.
When the deal was announced last month, Plexus chief executive Ben Van Bilderbeek said: “Plexus sees the share buyback as a practical and positive move that enables the acceleration of the delivery of the equipment into Russia, and positions Gusar ready to fulfil anticipated rental wellhead exploration orders in Russia, one of the world's three major hydrocarbon producers.”