I daily listen to Gayatri Mantra at work, at home and even while sleeping. I rarely chant this mantra. Does listening the Gayatri mantra have any effects same as chanting?
No, listening does not have the same effect as chanting. Listening is a step only. In the Brihadaranyaka Upanishad, Yajnavalkya say to his wife Maitreyi that there are three steps - hearing (sravanam), reflecting (mananam), and meditating (nididhyasanam). These three steps have been echoed by all the commentators. As Swami Vivekananda has pointed out, one does not learn chemistry by repeating the word 'chemistry'; one has to go into a lab, mix chemicals together and burn yourself a few times to learn chemistry. Physical, mental, ethical, and spiritual disciplines must be exercised. Listening is a good first step, but it is only a first step. What good will come of listening if your mind is running uncontrolled on worldly matters at the same time?
Narada says in his Bhakti Sutras 36-37 (Swami Prabhavananda translator):
(Supreme love is attained) by uninterrupted and constant worship of God.
By hearing of and singing the glory of the Lord, even while engaged in the ordinary activities of life.
Narada says this repeatedly throughout the Bhakti Sutras - not only hearing but also speaking.
Patanjali says in his Yoga Aphorisms I. 28. (Swami Vivekananda translator):
The repetition of this (Om) and meditating on its meaning (is the way).
So why is hearing not the same - not as effective - as praising, or chanting, or repetition? If you have two bells and you strike the first, it will vibrate and give out a loud sound. The second bell nearby will feel the vibrations from the first bell and start to vibrate in harmony a little but it will not give off any sound, and will stop vibrating well before the first bell that was struck. It is only by striking the second bell that a large vibration and sound can be had. Similarly, listening to the gayatri may startup some small vibrations inside a nearby person that are in harmony with it, but once it stops, the vibrations stop with no lasting effects.
Krishna says in Gita 9.34:
Fix your mind on Me, be devoted to Me, sacrifice to Me, bow down to Me. Having thus disciplined yourself, and regarding Me as the Supreme Goal, you will come to Me.
All schools of the Vedanta stress the repetition of the name of God. If you want to have your mind fixed on God it is only through your own practice. There is no shortcut.
To put in simple words, not only for chanting Gayathri mantra, I say this for chanting any sloka - the difference is just like film and books. When you listen to sloka or watch a film, we could just see the someone's feel when they enjoy rain shower, whereas when we sit in front of pooja room or enchant sloka in our voice is like reading a book, that is we ourselves feel the freshness and peace when we stand in rain shower.
Chanting helps in focusing concentration,improves breathe system of the body and the associated vibration acts as a positive points to the immune system. Listening needs more attention for achieving body and mind synchronisation. Hence listening and chanting both have their merits and demerits. Effect will be same if both listening and chanting done with devotion.
The mantras and shlokas are very carefully composed. Apart from the meaning and the tune, the experience and effect its chanting creates on body and mind is also considered. Your vocal cord that generates the sound is in direct contact with your body, so when it vibrates to create sound, it also transmits this physical vibration into your body which affects us in lot of ways. Try chanting Om mindfully. Observe your neck and your spine. It vibrates with the humm. Ask a flutist how it feels after playing some raaga.
Chant Gayatri mantra or atleast OM for some time and sit with eyes closed. Your body resonates under the vibrations of the chant. It is not possible to effectively recreate this experience when listening to it.