First thing is that, Guru is not equal to God, Guru Is the God. The question of equality comes when there are two different things, but Guru and God are non-different. So the Bhagavatam says:
acaryam mam vijaniyan navanmanyeta karhicit
na martya-buddhyasuyeta sarva-deva-mayo guruh [SB - 11.17.27]
One should know the acarya as Myself and never disrespect him in any way. One should not envy him, thinking him an ordinary man, for he is the representative of all the demigods.
The problem in understanding the identity of Guru and God is that when we think of the Guru we think of the physical body and qualities of the guru, but when the scripture (Guru Gita / Skanda Purana) talks about Guru being Brahma, Vishnu and Shiva himself it talks about the guru tatva or the essence, not just the physical guru. So the text defines the word Guru as below:
gukāraṃ ca guṇātītaṃ rūkāraṃ rūpavarjitam
guṇātītamarūpaṃ ca yo dadyāt sa guruḥ smṛtaḥ
The letter gu means guṇātīta (beyond qualities) and ru means rūpavarjita (without any form). He who gives this formless and qualityless state is said to be the Guru.
Since the text is talking about the essence of guru, not exactly the physical guru, there should be no confusion regarding guru's identity with the supreme gods. Otherwise there would be so many different Brahma, Visnus, etc. on earth as there are so many gurus and each of their disciples and followers address them as the God. So guru as a tatva or essence is same in all God realised Gurus even though the physical bodies differ.
Because Guru Himself is God realised or brahman vid (knower of Brahman), he is Brahman Himself. And from the point of essence whether we say Brahman, Vishnu, or Shiva all are ultimately but one. So the text says Guru is Brahma, Guru is Vishnu...
Just like God has a form, so also the guru has the physical form of a human. So pointing out the identity of Guru and the gods (Brahma, Vishnu and Shiva) from the point of view of the essence, the text explains how the Guru is different from those well known gods physically:
atrinetraḥ śivaḥ sākṣāt dvibāhuśca hariḥ smṛtaḥ
yo'caturvadano brahmā śrīguruḥ kathitaḥ priye
Guru is Shiva directly without the three eyes and is said to be Vishnu with two hands. O beloved (Parvati), guru is said to be Brahma without the four faces.
Now that we have this much understanding, we can answer the questions:
Why is Guru equal to brahma, vishnu and maheshwara?
In short, becaue Guru is someone who is brahmanistha and brahmavid, He is Brahman himself (brahmavid brahmaiva bhavati [Mun. Up. - 3.2.9]). And because Brahman in His different temperaments is commonly known as Brahma, Vishnu, and Shiva, the text also equates guru to those Gods. But the thing to keep in mind is that the text mostly talks about the guru tatva, not just the physical guru.
What is the exact meaning of "Guru" in this poem?
Guru can mean many different things depending upon how it is interpreted. The text itself gives different meanings of the word guru which are as below:
gukāraścāndhakāro hi rukārasteja ucyate
ajñānagrāsakaṃ brahma gurureva na saṃśayaḥ
The letter gu is indeed darkness and the letter ru is said to be light. There is no doubt that Guru only is the darkness destroying Brahman.
gukāro bhavarogaḥ syāt rukārastannirodhakṛt
The letter gu is like disease of wordly attachment and the letter ru prevents that. The guru is known as he who takes away the disease of wordly attachment.
gukāraḥ prathamo varṇo māyādiguṇabhāsakaḥ
rukāro'sti paraṃ brahma māyābhrāntivimocanam
The letter 'Gu' is the first syllable which expresses maya and its material natures (satva, rajas, tamas). The letter ru is supreme Brahman which releases from the illusions of maya.
Why is Guru so special in Hinduism?
Not just in Hinduism, guru is special in every case. But it is only Hinduism which states Guru as non different from God and teaches to respect Guru just like God (yasya deve para bhaktir, yatha deve tatha gurau). The reason is simple because our sages and saints had realized the ultimate truth and known the essence of all. They knew that the ultimate essence, known as God, can only be known upon the grace and instruction of the guru. So it is always instructed to get knowledge from the tatvadarshis or the knowers of the truth:
tad viddhi praṇipātena paripraśnena sevayā
upadekṣyanti te jñānaṁ jñāninas tattva-darśinaḥ [BG - 4.34]
Just try to learn the truth by approaching a spiritual master. Inquire from him submissively and render service unto him. The self-realized souls can impart knowledge unto you because they have seen the truth.
Since it is the guru who helps the seeker realize God in the first place, Guru is more important than God. Only after following the instructions of a guru that one meets God, so when both of them come at once, it is the Guru whose feet should be touched first.
Secondly, in Hinduism knowledge (vedas, puranas,etc.) has been obtained from one person to another in a successive order (guru parmpara) starting from Brahma. And Brahma himself got knowledge from the supreme God. So there is the statement Krishnam vande jagadgurum. When one will realize the truth he will be able to see how exactly Guru is God Himself and then the following statment of the text will make sense to him:
yo guruḥ sa śivaḥ prokto yaḥ śivaḥ sa guruḥ smṛtaḥ
- He who is guru is said as Shiva, He who is Shiva is remembered as Guru.