So... back to topic...
I tinkered with that Google Maps Extended Routes extension in FireFox and came up with mixed results...
Yes, it adds the "via points"...
No, it doesn't export a
working *.gpx route; opening the file on MapSource (or BaseCamp) just shows 'straight-line routing' from A to B to C to etc...
Not enough for the dense webs of the European road layouts, where C could be reached from B via 5 different roads, 4 of which you'd like to desperately avoid for whatever reasons...
So you have to export a track; opening this in MapSource or BaseCamp now shows the actual just created Google Maps route layout...
In BaseCamp I now could convert this into a route (containing 4~5000 track-points such can take a while), which of course lacks
any of the previously created via points; just [start] and [end]...
One unintended [recalculate] and your original plan avoiding traffic and roadworks is down the drain...
So better use your plotting tool/cursor in MapSource (BaseCamp) and create a new route on top of that shown track, avoiding the bloody logarithms, creating
your route as you had intended...
This you can then high-lite and send to your satnav...
As many of the oh-so-modern new Garmin units are notorious of running amok with unauthorized secret recalculations while down the road, some users transfer the track and the route onto their skittish devices, to display both while on the move for having visual confirmation of still being on the intended roadways, or if that little Android based devil has again silently dragged them elsewhere...
My 22 year old Garmin Quest-I is too plain for such whims...
As long as I not
purposely command it to [recalculate] it'll follow a transferred *.gdb route to the last dot... even with no via points set to force the routing over a particular road to circumvent any obstacles...
ze GF's Zumo 220 again tends to be a tad moody, it already has that [importing route...] feature(?!

), during which it definitely recalculates anything transferred from the PC, stubbornly following its internally set routing logarithms... so without
sufficient numbers of set via points the thing drives anywhere but the original route...
My conclusio:
Google Maps Extended Routes seems good for checking actual traffic situation across Europe and
exporting a track avoiding hold-ups as blueprint for your route... nothing beyond that...