I like the idea of jump gates, they make starflight easier in civilized
areas without making it so in not-so-civilized areas.
I like this interpretation:
Any vessel can enter a jump gate. If it has no jump drive, the
distance it will travel will depend on the gate's jump rating.
This rule lets player start out with a non-jump ship without requiring
them to do all adventuring in one star system. It makes the "entry cost"
for civilized-area adventures less (important if no one mustered out
with big bucks or a used ship) and gives them a chance to build up some
bucks for an upgrade to jump capability.
If a vessel primed for jump enters the gate, the NET jump will be
the vessel's jump rating, PLUS the gate's jump rating PLUS 1.
This lets a gate function as a "first stage" to kick a starship out
farther than it could get on its own, and thus deeper into adventure.
It also makes retreat a somewhat lengthier affair, since two jumps
(at least) will be needed to get back. The plus 1 added above gives
a bit of an inducement to use the double jump field, since you will
likely get further than with two separate jumps.
If you want the jump-6 limit to still hold, we can assume gates are
limited to jump-5. The net effect goes like this: the gate jumps the
ship to its destination (1 week) and as the ship materializes at the
destination, and before the jump field from the gate collapses, the ship
jumps again, using residual energy of the collapsing field. The ship
"stutters", jump-1 (another week) and finally the ship's normal jump
(three weeks total. The net effect maxes out at jump-12, (jump-6
ship approaching jump-5 gate) which takes 3 weeks, and costs just less
than two jumps of jump-6 (two weeks) in a jump-6-equipped starship.
At that limit, effectiveness is a judgement call. Below the limit,
(say, jump-3) the gate's effectiveness is plain.
Of course, the players will find a bill waiting for use of the gate when
they get back. Or maybe you charge them as they approach.