Marian Romans vs Gallic

One solution I like to use is Psiloi Skirmishers.
Just by their being out in front they can break-up a line of Warbands,
whether they recoil them, or stand on an equal score with 3Wb, or the
Ps recoil…and the Warbands can’t kill ‘em (they just flee if doubled).

Also, a line of Blades is still superior to a line of rear supported Warbands.
Combat factor 5 (needing doubles for a kill) v combat factor 3+1 (with a ‘quick kill’),
showing the chances out of 36, i.e. all the possible outcomes of both sides rolling a dice:-

                                  Wb Doubled   Wb Recoiled   Equal Score   Blade Recoiled/Doubled
Bd with 2 overlaps  =         12                 18                  3                       3
Bd with 1 overlap   =           6                  20                 4                        6
No overlaps           =           2                  19                  5                      10
Wb with 1 overlap  =           1                  14                  6                      15

Note the items in blue: most of the time the Wb will be recoiled, making it
relatively easy for the Blades to get an overlap…or even two overlaps.
Plus of course the Wb line, being two deep, will be shorter, exposing their
flanks…but the Bd will also be shorter as they need reserves to fill in any gaps.
Also, there is no rear support in Rough Going…bad news for the Wb, but fine for Bd.

(All this become easier to understand when using the following chart:-
static.wikia.nocookie.net/fanaticus-dba/images/e/e7/COMBAT_EFFECTS_CHART_for_DBA_%26_HOTT.pdf/revision/latest?cb=20190204191800 )

Basically, Warbands don’t like overlaps…but they recoil so often that they just can’t avoid it.
Having Psiloi-Skirmishers out in front breaks-up their nice neat Warband battleline.

I am luck that I only played a couple of games in 2.2 so 3 was really where I started my DBA gaming so cant get caught out by the differences.

However a Wb following up a destroyed Bd does need it's mates either side to do the same,
otherwise it's looking very dicey...for the pursuing Wb!

So it's all swings and roundabouts really.

Depends a bit where the Wb's mates are and who has the next bound. (I assume you haven't got solid 6x2 block of Wb. Even if you have, there is a good chance the elements at either end of the line will have the door closed on them by some of the remaining 6 Roman elements.)

Only if the Romans have a unit behind the line that can contact it.

Basically, as menacusseccundus says, the Wb general has to decide, do I double up my Wb in the hope that the slightly extra chance of destroying the Bd's wins me the battle before the Romans get around my shortened battle line and start destroying the flanking troops or taking out the camp etc.

The Roman also has other options;
As previously mentioned, using Ps v Wb. The Wb don't get the supporting rank but if in two ranks the second one is pinned and the Ps can't be destroyed.
Use Cav. The Cav are 3 v Wb 2 and again the rear support doesn't count but is pinned.  
Use Ax, they are more likely to be recoiled against double ranked Wb but less likely than Bd to be killed outright.

The nasty combination is when Dacians bring along some Sarmatian allies.

Overall though I think most of the advantages are with the Romans but occasionally the Wb may get lucky and score a famous victory (does that sound like history?)

I agree 100% with you Sheffmark.
Much depends on positioning, length of battleline, reserves, terrain, and just pure luck!

Were I to make a very rough estimate, I’d say the Bd army would beat a Wb army about
two times out of three, and a Bd army lose about one in three against a Wb army.

It’s this that makes the Gauls/Britons/Germans/etc such a threat to Rome.

Nice little report