I've been playing SW2 for several missions now, and I still can't access the trials. There's a gate blocking the opening next to Larry's where the pathway up to the trial area is, and I can't open it. Do I have to do something special before the gate opens, or what?

They should unlock after fighting a specific boss (I don't want to accidentally spoil the game for you if it's your first time playing it, so I won't go into detail there). One will unlock after you finish the 9th mission in the story, for example.

If this isn't your first time playing it, then they should all be unlocked on a NG+ run.

Aha, that explains it. Thank you!