when I'm playing zerg I have almost always a problem of low gas and lots of minerals. Especially it happens when playing with terran because I need mutas + banelings. How to spend minerals correctly? Maybe expanding just for gas is an option?
The short version is: You're backing up on Minerals because you don't have enough Larva for your Mineral Dump (ie. Zerglings). You need more Hatcheries, but not more Expansions. Put the new Hatcheries in your current bases (3 Hatch on 2 Base is a nice ratio, but 5 on 3 is nicer).
I'm going to use some terms here to help you understand the solution to your problem and I think I should define them before I use them:
2.5 is sort of a funny number to choose. However, a fully saturated base should produce 2.5 Minerals for every Gas.
There is a reason you see people go Roach/Hydra or Muta/Ling and not Hydra/Muta or Roach/Ling (at least not after the early game). The reason is that these units aren't just synergistic in combat, they're also synergistic in production. Usually, when planning the force you want to take to battle you want to plan a "Money" unit. This is a unit who you're bringing along to do the heavy lifting. To help prevent you from just massing Ultralisks or Thors, these "Money" units are Gas Heavy. As a result you need to plan a Mineral Dump, a unit to throw those excess minerals at. While expansions are great Mineral Dumps they suffer from the problem of needing to be defended. To combat this you need to plan your army intelligently and not just build units because they're "cool".
One of the big problems Zerg in particular runs into is that their basic Mineral Dumps are Zerglings (I'm ignoring Roaches for the moment which are also a great Mineral Dump but don't suffer from this problem). Zerglings have the unique property in that they're Larval Expensive. For 100 Minerals of Zerglings you need 2 Larva (by comparison the numbers are 1 for Roaches, .6 for Hydras, .5 for Muta and .25 for Ultralisks). This means to field and maintain a large Zergling Force you need higher larval production than you would otherwise get. Often time if I'm running Muta/Ling I'll go 3 Hatch on 2 base. This is probably what's happening to you:
Your prioritizing Larva for Mutalisk (a smart decision) and not having enough left over for Zerglings.
The solution here is not to expand but to place those extra Hatcheries (and Queens) inside your current bases so that you have enough Larva for a larger Zergling force.
On the same topic Banelings are actually fairly Gas Heavy. This tends to get over looked because you get 4 for 100 Gas but it is true. The reason you get away with Muta/Baneling is because you're actually building Muta/Ling and sacrificing Mutalisks for Banelings (a smart choice against a Marine Heavy opponent). So don't forget to build those Zerglings!
Expand when you can afford it and make sure that you get enough gas for your plan.
Any excess minerals can be used for Zerglings and Overlords,
If you want to spent some gas too, the gas ratio of Roaches, Hydralisks and Ultralisks are perfect.
yes, this is the most common problem with Zerg - you do get a lot of minerals and starve on gas. The best way to overcome this is to expand and prioritize gas over minerals at the expansions. The most common way I do this, is when I have 500/600 minerals, I try to expand and make the gas extractors first, once the bas is up and running I start harvesting minerals from the new expansion. If you still have a problem with minerals even expanding enough to get 2/3 bases, spend the money on zerglings / roaches to keep your minerals low and gather enough gas to go mutalisks.
One more thing, going muta from 1 base is really risky as you won't have enough units to defend early rushes. Try a common 2 base muta build when expanding really early - 14 pool, 15 hatch.
Hope this helps!