"Gantry" is a more contemporary term, usually used in reference to studios or movie sets. If you're talking about a theatre built to produce live entertainment, particularly one dating back to the Victorian era, the area above the stage is called the flies (because that's where drops and other scenery "fly" in from). The fly floor is a platform from which the system of ropes and counterweights responsible for the raising and lowering of scenery are accessed. It's usually located in one wing, well overhead. In the pre-automation days, the flies were operated by hand, by stagehands who were often ex-sailors and accustomed to both heights and ropes. To access the fly rails (the long horizontal poles from which flown scenery is hung), stagehands would use catwalks. The catwalk would be the logical place for a fatal fall to occur, especially if someone has tampered with the safety railing.
The proscenium is essentially the opening through which the audience views a show - the "picture frame" for the show, if you will. If you think of the stage as a box, and take away one side of the box, you've created a proscenium. The playing area downstage of the proscenium is called the apron. (If you are headed downstage, you are moving toward the audience. If you're headed upstage, you're moving away from them. This is because most early stages were raked, meaning that they were built to slope downward toward the audience. This improved sightlines for the audience but was hard on actors since they were essentially playing at a tilt; theatres in the 20th century were typically built with raked audience seating instead.)
Stage left and stage right refer to the actor's left and right when facing the audience; house right and house left refer to the audience's left and right when facing the stage.
In the 1850s (when your story is set), electric lighting did not exist in theatres. Gaslights illuminated the stage, and most of them were positioned in footlights (a horizontal row of lights along the edge of the apron, between the actors and the audience), border lights (a horizontal row of lights mounted overhead between the fly rails), and wing lights (vertical strip lights focused on the stage from the wings). Gaslight was smelly and generated a lot of heat. Limes were forerunners of the modern spotlight; by burning a cylinder of quicklime, a sharp, clear beam of focused light could be created to illuminate an actor's face, a particular part of the set, or even make the illusion of the sun or moon in a scene. (It's where the expression in the limelight comes from.) Limes were typically located in the theatre's balcony, or at the rear of the house.
In theatres where music was needed, a pit was located just in front of the apron, between the stage and the audience. This was a lowered area in which the musicians sat to play. The pit might be surrounded by a low curtained rail to mask the musicians from the audience.
The wings are located off stage right and off stage left. You can certainly refer to them in the singular ("The stagehand stood in the left wing."), and certainly both the right and left wings were used. The crossover is the area between the set and the stage's back wall where an actor or crew person can pass unseen during a show (not every set design provides this option). During the early Victorian era, settings were established by using painted drops rather than the huge, realistic sets you see in contemporary theatre. The wings didn't need to be spacious; therefore the wing area was usually quite narrow, cramped and dark. (Theatres constructed prior to the 1930s are problematic for today's productions; the show I did last year was in one of Broadway's oldest theatres, and wing space was nearly nonexistent. All our massive set pieces had to be "flown" when not actively in use, which meant that if you looked up as you exited into the wings, you had the unnerving experience of seeing a staircase, a king-sized bed, desks, department store display cases and the like dangling above overhead.)
Hope this info helps.
Great discussion. 
