According to official 40k "fluff", interstellar starships of the Imperium use plasma drives, which can take up to one-third of the length of the ship. Plasma drives are composed of drive tubes, engine compartments, and plasma reactors. The question is: how do we convert this concept into a workable GURPS Vehicles 2e stardrive design option?

First we can look at page 36 of GURPS Vehicles 2e and check-out some of their "plasma" (fusion) stardrives:

TL   Type              Weight                  Fuel Usage  Power
9    Fusion Rocket     (0.05 x thrust) + 50    0.02W       0
9    Optimized Fusion  (3,000 x th.) + 20,000  0.004H      0
10+  Fusion Rocket     (0.025 x thrust) + 25   0.02W       0
10+  Optimized Fusion  (300 x thrust) + 2,000  0.004H      0

Fuel Consumption:
W is water and H is hydrogen, both in gph (gallons per hour).

As you can see the above options are not really suitable as they require ridiculous amounts of reaction mass. A more appropriate option would be to use the reactionless thrusters on page 38 of GURPS Vehicles 2e with power consumption replaced with a fuel consumption of at least 0.00012M (as per page 13 of GURPS Vehicles: Expansion 1)

TL   Type               Weight         Fuel Usage            Power
9    Standard Thruster  1 x thrust     0.00012M to 0.00024M  0
10   Standard Thruster  0.5 x thrust   0.00012M to 0.00024M  0
10   Super Thruster     0.2 x thrust   0.00012M to 0.00024M  0
11+  Standard Thruster  0.05 x thrust  0.00012M to 0.00024M  0
11+  Super Thruster     0.02 x thrust  0.00012M to 0.00024M  0

Fuel Consumption:
M is pounds of any mass; in this case M would be hydrogen, of which 1 pound equals ~1.72 gallons. In short fuel usage will be between 0.0002H to 0.0004H (gph).