The Germans were still using horses to a great extent for moving supplies and equipment around, while the Allies (Soviets included) were mostly using trucks. It's possible to protect a soldier from the effects of gas at least to a certain extent; but a horse is always going to be vulnerable especially when it comes to something like sarin which is absorbed directly through the skin. First use for the Germans would have left the other side's trucks intact while the Allied retaliation would have really put a dent in the German's horseflesh.

Also, most of the German R&D facilities for gas weapons were in occupied territory; and so by the time Germany got really desperate they'd already been overrun by the Soviets. Nerve gas is so incredibly poisonous that one of the biggest challenges the weapons designer faces is how to make it in any quantity without accidentally killing your whole work force. Restarting production inside Germany on short notice would have been challenging to say the least.

As far as the Soviets go, I'm not aware of any sources detailing their discussions about first use. However, during the early part of the war when desperation might have made first use seem attractive the Germans had a huge advantage both in bombers and in heavy artillery. It doesn't make a lot of sense to use gas weapons first if the enemy retaliation is going to be able to throw substantially more back at you.

Once the Soviets gained the initiative on the ground gas weapons still didn't really make much sense for them. Their goal was basically to steamroller west as rapidly as was feasible in order to keep the Germans off balance and to put themselves in the best possible position post-war. The use of gas slows things down tremendously as your soldiers now have to fight wearing masks and chemical suits, have to be decontaminated before they can move outside of the area where gas was being used in order to eat and what not, and you also have to work out some way to keep moving supplies forward through the areas where gas was used. Even after they were in a position to gas the Germans without the Germans being able to do much back gas would still have been a loser for the Soviets.

I've got to head off to work, but I hope this little bit helps.

In addition to the previous answer: any use of chemical weapons slows down the attack, since they just poisoned the area they are now supposed to blitzkrieg through. This makes gas (marginally) useful for slow trench warfare, but useless for motorized offensive.