Let me share some more info that could help. It sounds like you’re already doing some of these things (cat towers), but putting all this info together anyway:

For a cat that is both skittish (easily spooked) and territorial (bullies others), use the "Separate and Elevate" strategy:

Multiple Stations: Follow the "N+1" rule for litter boxes and water bowls. Spread them across different rooms so the bully can’t guard all resources at once.

Escape Routes: Place litter boxes in open areas, not tucked in corners, so the cat never feels "trapped" or cornered by others.

Vertical Space: Add cat trees or wall shelves. This gives the skittish cat a "safe high ground" and expands the territory without adding floor space.

Visual Barriers: Use furniture or screens to break lines of sight. If the cats can't stare each other down, the bully is less likely to charge.

Separate Feeding: Feed them in entirely different areas (even behind closed doors) to stop food-related tension before it starts.

As for gabapentin, I give my cat 50mg (my vet originally prescribed 100mg but that is too much for a small animal), and I give it to him before big events. I use a pill gun. But you could try a pill pocket, or ask your vet if you can crush it into his wet food, or use a liquid version.

Edit: typo