Did you add your sugar yet? Maybe add 2-4 quarts of RO (depends on if the gravity does/does not include the sugar) to the fermenter, swirl it around, call it a day. Careful,with sanitization of you do this, swirl quickly, and have the water at a similar temp to the wort. Then, mostly your later hops may be impacted, but I bet it will still be pretty good.
Also for next time, refractometers are relatively cheap....they help track your SG during the boil, so you can reduce the flame if you're way ahead. I'm doing this beer next myself. If you like it, brew it again, then do an exbeeriment to see if you can taste the difference with the diluted 1st batch! I totally screwed up my first stout BIAB, fell about 8 points short on FG, and now I enjoy it as a dinner beer, kinda like liquid sweet bread as a side! Still tastes good.