I didn't find much on Google about the Planinc Variation except that it's named after GM Albin Planinc, who championed it in the 1970s and then in the 1990s a German correspondence player named Michael Schirmer experimented with it. I was unable to locate any of Schirmer's games.
In the book The Alekhine for the Tournament Player by Lev Alburt and Eric Schiller they say that players of the white side who play the Four Knights Variation need to be prepared to meet 5...g5 (the Planinc Variation) because the right moves are not easy to find at the board and they amusingly added that this is the only thing that the variation has going for it.
The only line I found that seems to offer black anything near equality is the following:
1. e4 Nf6 2. e5 Nd5 3. d4 d6 4. c4 Nb6 5. f4 g5 6. exd6 (This is white's most popular continuation, but engine analysis indicates that 6.Nc3 might actually be better.) cxd6 (Black almost always plays 6...Qxd6) 7. fxg5 Nc6 8. Nf3 Bg4 9. Be3 Bg7 10. Nc3 Bxf3 11. gxf3 O-O 12. f4 d5 reaching the position shown in the diagram. Using Stockfish I ran a Shootout from this position which resulted in all five games being drawn.
My analysis did not discover anything that would indicate that this variation might have some hidden resources for black that would make it worth playing. In fact, it seems to confirm that if black plays 5...g5 he is taking a huge risk and it seems that unless white plays very poorly black will immediately be on the defensive...not what he wants when he plays a gambit!
As for the players, I did a post featuring one of Planinc's brilliant wins involving a Q sac a few years ago HERE.
Georgi Tringov (March 7, 1937 – July 2, 2000) was a Bulgarian GM. He was awarded the IM title in 1962 and the GM title in 1963, the year he won the Bulgarian championship. Tringov was active mainly during the 1960s and 1970s and qualified for the 1964 Interzonal where he finished fifteenth.
Tringov had numerous successes in international tournaments, including first place at Vrsac 1973. He placed fifth in the 1955 World Junior Championship and played for Bulgaria in five World Student Team Championships (1957 through 1960), winning the individual gold on board four in 1957 and 1958. Playing on board 2 in 1959 he scored +11=2−0. Tringov played in 12 Olympiads and in 1968 he won the gold medal with a score of +8=6-0 on board two. In 1978 he scored +6=5−0) on board three.
Tringov was involved in a controversy in his game against Korchnoi at the Olympiad in 1972. Their game was adjourned after 41 moves with Tringov to seal. his next move. Sealed moves were written on a separate piece of paper, not the player's score sheet. But Tringov sealed his move by writing it on his score sheet. Upon resumption, when the arbiter opened the envelope it contained Korchnoi's score sheet but not Tringov's. The arbiter ruled the game a forfeit win for Korchnoi. Oleg Neikirch, the Bulgarian team captain, protested but the arbitration committee upheld the arbiter's ruling. After the Olympiad was over, it was learned that Tringov had accidentally placed his score sheet in his pocket. Tringov discovered his mistake several days after his forfeit but was too ashamed to admit his mistake to the organizers of the Olympiad.
If anyone is interested trying this variation I have made a pdf copy of this game that can be printed out for reference. Download from Dropbox.