Messerschmitt Bf.109G, Romanian users

The Fortele Aeriene Regale ale României (Royal Romanian Air Force) adquired some Bf.109Gs in March 1943 with the 7th Fighter Group being the first unit to be equipped with them. As that unit had previously flown the Bf.109E, the new aircrafts were delivered directly in the front, as part of the JG 3 'Udet'.
The first mission flown by a Romanian Bf.109G took place on 29th March in a mission that reported both the first casualty and the first victory flying a Bf.109G. The 7th Fighter Group (Grupul 7 Vánátoare) achieved many victories over southern Ukraine until they were replaced by the 9th Fighter Group (Grupul 9 Vánátoare) which took over their aircraft and some of their most experienced pilots.
The Grupul 53 Vánátoare (53rd Fighter Group) was also equipped with Bf.109Gs and they achieved 2 confirmed kills on 1st August 1943 during the Operation Tidal Wave at the price of one damaged machine.
By summer 1944 both Grupul 7 and Grupul 9 were joined together after being partially re-equipped with the Bf.109G and took part in the defence of Bucharest against USAAF and Soviet Air Force's raids. As Romania switched sides on 23rd August 1944 both groups were sent to defend Bucharest against Luftwaffe's raids scoring some victories against German bombers and transports.
The remnants of both groups formed the 7th/9th Fighter Group and were soon renamed as the 9th Fighter group. They were sent to the Transylvania front, together with the 44th Fighter Squadron which was equipped both with Bf.109G and IAR-80.
The first combat between German and Romanian Bf.109Gs took place on 18th September with German victory as they damaged the aircraft of slt. Andrei Pop. As the 44th Squadron was relegated from the frontline in late 1944, the 9th Fighter Group was practically the only Romanian fighter unit, as the IAR-80 of the 2nd Fighter Group were assignated to ground-attack roles. In February 1945 they were joined by the 1st Fighter Group which was also equipped with Gustavs.
Overall, Romanian Bf.109Gs remained in the frontlines until the very end of the war achieving a considerable amount of victories, some of them against the more modern Bf.109K variant, which was the last, and best variant of the Bf.109.
Most of Romanian aces scored their victories with the Gustav, which was highly praised. Lt.Av. Teodor Greceanu, with 20 victories, said about it that "it fits me like a glove" and Cpt.Av. Serbanescu thought that it was better than the Mustang, however, he was proven wrong as on 18th August 1944 he was shot down and killed by several Mustangs.
It's worth to tell how one of the best Romanian pilots, Constatin Cantacuzino, flew a modified Gustav (depicted below) to Foggia, in Italy (which was controlled by American troops), with Lt.Col. James Gunn III as passenger (the highest-ranking American prisoner on Romanian soil) in order to return to Romania with 56 Boeing B-17s to airlift the 1274 US POWs on 23rd August 1944. He returned flying a P-51 Mustang as his Bf.109 couldn't be refuelled.
It was by far the best fighter the Royal Romanian Air Force had and also the most effective and the only one to achieve kills against the Mustang. After the war, the Gustav was still in service until 1951 serving in mixed squadrons together with the Lavochkin La-9 and it was replaced by the MiG-15.

Sources:
1. https://www.worldwar2.ro/arme/?article=400
2. https://en.wikipedia.org/wiki/Constantin_Cantacuzino_(aviator)
3. https://en.wikipedia.org/wiki/Messerschmitt_Bf_109_operational_history#Combat_service_with_Romania
4. Salamander Books - The Complete Book of Fighters