Unlike the common misconception, gas ape pyro is not the "most efficient" class in 2 cities especially if you have room temperature IQ like the usual mvm player. This guide will guide you through a 3 point process that is meant to guide you away from this bad class.

Time for lecture.

(top img from https://steamproxy-script.pipiskins.com/id/hat_wearing_man/ he wanted to be known as THE gas ape)

If you graph the dps of the gas passer based on money you end up with a curve that is a flatline with a single peak.
The peak is caused by explode on ignite apegrade at 400 credits.
After the 400 credits your weapon is unable to do more damage.
Now if you were to upgrade your primary weapon to do damage while your gas is off CD which is extremely rare in the average gas ape pyro I see (most gas apes spam airblast for 60 seconds while waiting for gas recharge) but if you are one of the smart ones then you can do up to
wait Ill google this
like 150 dps according to some ppl.- (non crit with perfect particle timing to gain max dmg [if you want an example of aim as pyro which people dont understand, think of the way you have to aim very specifically on tank to do 78 splats on phlog with krit instead of ♥♥♥♥♥♥♥♥ damage)
You can upgrade damage 4 times on your primary weapon costing 400 credits each.
This will cost you 1,6k credits
When added with the gas upgrade your damage flatlines after 2000 credits unless you can spam krit canteens which you cant because 90% of engineers sadly suffer from (acquired immunodeficiency syndrome) and they dont buy 2way teleporters.

Now we understand the relation between money in mvm and the theoretical damage limits of the pyro class in mvm.

In 2 cities there are 4 missions:
in hattan1 you start with 900 and get 1100 on w1 with A+, meaning your "most efficient class in mvm" has reached its maximum theoretical upgrade limit and hence the theoretical maximum damage limit (this mission caps at almost 9k credits so you essentially waste 7k creds on upgrades that dont impact your robot killing speed
in hattan2 you start with 800 credits, get 950 from w1 and then 1k from w2, the start of w2 already gives you 50 from fister heavy and 200 from demos so you reach your peak there. Max credits here? Almost 7k
in r1 you get 700 start 900 w1 900 w2, 400+ w2 from 5gigapyros before tank so you get upto 2k there. Max money almost 7k.
rotten2 starts with 1200 you get 1100 credits on w1, again peaking ur dmg. max almost 9k

now the third and last concept you must understand to see the big picture is "why does damage matter?"

Damage matters because the objective / the way to win in mvm is to kill everything.
If you kill slow you win slow
if you kill fast you win fast

If you play pyro your kill speed will be on the last wave the same as on w2 in 3/4 and w3 in 1/4 missions 2cit
Other tours dont matter they are not very good :steamhappy:

This is why you do not play gas ape pyro in mann vs machine game mode. maybe like this excellent steam guide on the topic if you learned something or you think people might learn from this

If you want to continue the learning process on learning what the best available class in mvm is for 2c consult one of the 100 beggars bazooka guides available.