Gas isn't really a breakthrough weapon in V2 though, assuming roughly equal tech levels the other side will soon develop gas defense, so after a few initial surprise attacks the effect of gas is fairly minimal unless you are fighting lower tech nations. As for tanks, I've always thought that the scale of provinces means that battles are assumed to include both the break in and breakout, that is you only take the province if you manage a successful breakout, so why you failed to breakout doesn't really matter, it's abstracted, all that matters is that you did.
I'll just add that gas attacks against people without gas defense isn't about offense or defense, anyway. The +3 to combat die rolls means that the side with gas inflicts punishing casualties on the opposition. It can be utilized effectively by anyone, regardless of conflict.
In fact, the irony is that the rush to get gas attacks is usually more important so that you get gas DEFENSE. If everyone already is getting gas defense, I don't care if I'm using gas attacks. I just want immunity to other people's gas attacks.
I think its interesting to note that gas and tanks were deployed in significant numbers during Nivelle. What ultimately let down Nivelle was the artillery plan, capability, support and co-ordination in combination with a number of other issues.
Correct me if I'm wrong, Mowers, but when either side could get 5:1 or 10:1 odds in terms of artillery in a particular sector, they did fairly well. It's just that they didn't have the kind of mobile artillery we have these days, and couldn't coordinate that artillery with radio for choosing new targets during an advance like we can with radio.
They also didn't have the sheer number of guns that we can often employ in Vic2. I mean, I can easily get 1:2 in terms of artillery brigades to guards/infantry. That's a ton of firepower to concentrate in a single province when you start stacking 150,000 troops in a single battle.
As for tanks and planes in Vic2, most of this discussion is irrelevant anyway. I don't find tank brigades (with pre-1920 techs) to be cost effective for the firepower they bring to the table. Post 1920, they get much faster, and you can pair them planes to get high recon/high speed/hard hitting formations (that are still a bit undergunned compared to post-1920 artillery brigades in raw scores, but at that point the discipline of the tanks makes a bigger difference). But at that point, we're talking about implementing WWII kinds of doctrines into warfare, just doing it a little early. Pre-1920, just build MOAR artillery and watch your width.