I'm wondering can I detect temperature, humidity and gas at the same time using DHT11, MQ-X sensors? If so, how? I can find numerous solutions using each of them separately, but not simultaneously.
Yes, you can use both sensors at the same time on the Raspberry Pi.
The DHT11 uses just one GPIO to trigger a reading and return the result.
The MQ-X sensors return an analogue signal and require an ADC (Analogue Digital Converter) to be read by the Pi. Typical ADCs use the I2C bus or the SPI bus on the Pi.
You need to choose solutions for both devices from the many available on the web. Choose ones which make most sense to you. You then need to merge the two solutions. If you have problems ask a (new) detailed question and we will help.