Gelfand is a 1.d4 player.
Kasimdzhanov plays the King's Indian, the Queen's gambit declined, and the Nimzo Indian against 1.d4.
Against the King's Indian, Gelfand likes to play the main line, sometimes with 7.Be3.
Both players have a very good record with their side of the Kings's Indian, and I hope we are in for a treat. After the damage Radjabov did in Wijk, I am eager to see more.
But maybe Kasimdzhanov is going to play more solid. After 1.d4 Nf6 2.c4 e6 Gelfand usually plays 3.Nf3. Kasimdzhanov never plays the Queen's Indian, but continues with 3..d5, transposing to the Queen's gambit declined.
Gelfand then usually plays the Queen's gambit declined with Bf4, or the Catalan.
Kasimdzhanov usually plays 1.e4, but sometimes mixes in 1.d4.
Against 1.e4 Gelfand plays the Najdorf, and sometimes the Petroff.
Against the Najdorf, Kasimdzhanov most often uses the English attack with 6.Be3. Gelfand has played all main moves 6..e5, 6..e6, and 6..Ng4 here.
Kasimdzhanov has an excellent record with the English attack. Therefore, and especially in a match situation, I expect Gelfand to play the Petroff, unless he is behind in the score.
In the Petroff, Gelfand plays the line with 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 Nxe4 5.d4 d5 6.Bd3 Nc6 7.O-O Be7 8.c4 Nb4 9.Be2 O-O 10.Nc3 Bf5, which is very solid and a good drawing weapon.
Kasimdzhanov will need an excellent preparation to generate winning chances with White against Gelfand.