Gordon is the one who causes the resonance cascade that leads to xen, xen is where the headcrabs, ant lions, vortigaunt etc live. The machine was sabotaged by the G-Man. This causes a portal storm all over the black mesa facility that lets the aliens through into our world and run amuck. The marines are sent in to kill the aliens and all the science personal to quiet it. It goes wrong like in Aliens. So they send in the Navy Seals to nuke the facility. The G-Man also tasks one of the better surviving marines, Cpl. Shepard with hunting down Gordon and murdering him before he can get out, he's testing Gordon to make sure he's good enough. Gordon escapes by getting through Kleiner's portal to Xen, after which hes captured by the G-Man and imprisoned until HL2 so it's absolutely necassary story.
and while those vids are ok, these are the best ones if you havent seen them.
https://www.youtube.com/watch?v=ZqMmYq0QjEA
https://www.youtube.com/watch?v=4lyUNs7eNhs