Thanks for the quick response. Don't plan on more then 100# on the rack, plus the Road Shower. Struggling with, while it may look cool, the reality ten months out of the year it will be bare, and of course not that many miles when in town.
I'm not clear from your original post if you re asking specifically about a Gobi or just musing about a rack in general and what sort of negative MPG might come from having a rack?
If you are not set on a Gobi but just generally inquiring, the it sounds like one of the lighter and cheaper basket / racks that is more easily removable might be the ticket for your needs? I have a Yakima basket which, once set up for the first time, can be put on and taken off by one person and takes about 5 minutes to tighten down with no tools.
I paid $300 for it, the middle extension, and the locking mounts on CL.
The Yak still fits (with 1/4" to spare!) in my garage (2.5" lift and 285s), but I take it off when not in use just to keep it from rusting, and I hang it from the garage rafters using 4 bicycle hooks. If 10 months out of 12 you aren't going to be using it anyway, no point in leaving it on all the time, and no point in buying a high dollar unit whose construction pretty much necessitates leaving it on all the time.
Downsides, it's not full length like some racks, it's not as cool as a Gobi or whatever, and it can't support a RTT (which I don't care for anyway, so no loss for me).
Upsides, cheap, easy to install, lifetime warranty, easy to find on the used market, hauls your specified load (100 lbs of boxes) just fine, no obvious MPG change (empty), elevated sides make it easier to secure gear.