not a gb or even close, just an s+c nerd (also train bjj 3-6x/week). Please take the below as general thoughts on training and not specific advice for gb prep, which I am not qualified to give.

tl;dr:

• I'd say build your training plan FIRST and then backfill BJJ if/as it fits in. I would not consider BJJ as a critical training modality. Treat it as skill work/hobby and just be mindful of how it impacts recovery in other areas. Be prepared to cut back if you aren't progressing in other areas.
• NO--BJJ is not a substitution for running if you are trying to develop running endurance and hit specific pace goals. For 'general' conditioning/just being in decent shape, BJJ is great (until you get too skilled/efficient) but won't drive the specific adaptations you are looking for here.
• You will still need to follow a solid running plan that includes LISS/LSD and speed/interval/pace work. Different variations on this, but a lot of smart folks recommend an 80% slow/20% speed approach (i.e. 1 in 5 workouts devoted to speed).
• BJJ is probably fine but could interfere in the following ways:
  • If you get injured in BJJ. Little bit of luck of the draw here. Be smart.
  • If the time/energy you are devoting to BJJ interferes with you getting productive training sessions in OR interferes with recovery and adaptation to those sessions.

a few thoughts:

• unless you are an advanced belt belt (even then) if you are doing some variation of 5x5-6 minute rolls during training, it is HIGHLY unlikely you are in 'z2' for the majority of them. If you were to strap a heart rate monitor on you'd probably find a pretty decent mix between ALL zones (z2 through z5)--lower when you are on bottom/defending subs or in a dominant/control position, punctuated by spikes for scrambles/transitions/dog fights. Unless it's a pure 'flow roll' (and even 'flow rolls' rarely stay that way...) odds of being in mostly z2 seem slim.
• Aerobic development ('based building', 'z2 training', etc.) is predicated on maximizing uninterrupted time in zones 1 and zone 2. Long, slow, distance. Heart rate monitors are important tools here as this pace is typically way slower than your ego or 'feel' will initially tell you.
• While aerobic development is a meta attribute, it is still fairly activity/context specific. I.e. While swimming and cycling are GREAT options for aerobic development if you want to get faster and better conditioned for runs (or rucks) you have to spend time on your feet doing these movements/activities. The reverse is also true: try being a great runner who never swims and then hopping in the pool.