I've gone through the level, turned on the generator, all that, but the gate that I am supposed to lead the big monster down doesn't open. I checked all the walkthroughs and done everything that they said, but the gate still doesn't open. Is this a glitch?
I think you are supposed to get on the train, and go at full speed towards the monster and then jump off. the train should slam the monster into the gate and blow it up, killing the monster in the progress
... no you're not.
Firstly, confirm you've actually done everything needed to turn on the generator. Destroyed the crates blocking the machine piston? Pressed the switch in the control room just above the piston?
Otherwise, it's probably a glitch... you could either load from a previous save, or noclip through the barrier to move on.
By the way, the Gargantua is killable if you throw enough explosives at it (grenades, tripmines etc).
Half-life is one of the best games I've ever played. All hail Gordon.