@p7eaven
Cribs to Purton, drop down to Easter Compton, then NCR 4 to Olveston, then NCR 41, Oldbury, Berkely and cut down to Purton if on MTB/Gravel and on to Glos. Lanes are generally great, only muddy around Hurst Farm when wet.
If your on a road bike I would just stay on NCR 41 all the way to Glos, only a mile on canal bank from Slimbridge but hard gravel so ok, then flat lanes through Frampton, Longney, Elmore to Rea Bridge and on to surfaced canal bank up to the Docks