Leko - Gurevich
Peter Leko is primarily a 1.e4 player, but has been using 1.d4 for variety, depending on the tournament situation and opponent, since his match against Kramnik.
Gurevich has a very consistent opening repertoire against 1.e4. He normally plays the French, but if he wants to play more risky, he plays 1..d6. In the candidates match I expect him to play the French unless he is behind in the score and getting desperate, as Peter has an excellent score against 1..d6 systems.
After 1.e4 e6 2.d4 d5 Peter Leko always plays 3.Nc3, and Gurevich always counters that with the classical French, 3..Nf6, after which Leko has always relied on 3.Bg5. For an important match such as this, he may also prepare the important alternative 3.e5. After 3.Bg5 Gurevich usually plays 3..dxe4.
Gurevich has a lot of experience with the French, and it will be important for Peter to prepare some good lines against it.
Gurevich - Leko
Gurevich is primarily a 1.d4 player, but he also uses 1.c4 regularly.
Against 1.d4 Leko plays the Nimzo or Queen's Indian.
Gurevich plays three different systems here, the Petrosian system against the Queen's Indian (1.d4 Nf6 2.c4 e6 3.Nf3 b6 4.a3), and the Classical or Rubinstein variation against the Nimzo Indian (1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 or 4.e3).
I expect Gurevich to play 1.d4 unless he is behind in the score, when he will switch to 1.c4.
Leko must be considered favourite in this match, but Gurevich is very experienced - a successfull opening preparation should be a key for Peter to win this match.