Method #1
Kill Roger while he is in the integration room by shooting him through the observation glass. 

Method #2
Kill Roger by shoving him through the glass.

Method #3
Kill Roger by planting a C4 on the airtug.

Method #4
Kill Gabriel by shooting a crate above him.

Method #5
Kill Gabriel and Roger at the same time by shooting the gas tank and setting them both on fire.

Method #6
Snipe both Gabriel and Roger silently without causing alert

Method #7
Kill Gabriel and Roger by posing as the pilot and intentionally crashing the plane or blowing up the plane with a C4

(Gabriel and Roger both leave the airport in 9 minutes)