Terminology
The Gift of the Holy Ghost can be a confusing topic in The Church of Jesus Christ of Latter-day Saints, not because the doctrine is especially complex, but rather because we don't have well-defined terms.
For example, in the section you quoted, the following terms were used:
- Holy Ghost
- The Power of the Holy Ghost
- Revelation
- The Gift of the Holy Ghost
- The Spirit of Revelation
These terms are sometimes used interchangeably, especially in historical texts, but they are also sometimes used to mean very different things. This can make it a very difficult topic to study. I could add to the above list a number of other terms that are also used when discussing the Holy Ghost that also have ambiguous and overlapping meanings:
- The Light of Christ
- The Comforter
- Gifts of the Spirit
- The Spirit
- etc
I bring this up because I want to make sure we are both speaking the same language before answering your specific question. So let's define some terms. I think these definitions are generally accepted in the Church, but as I said, sometimes they are used in other ways and I want to make sure my answer is clear. Hopefully this helps explain the quote a little bit too.
The Holy Ghost (AKA "the spirit", "the holy spirit", or "the comforter") is a non-corporeal being, distinct from God the Father and Jesus Christ but united with them in will and purpose. He is the member of the Godhead that we know the least about. You can read more about Him here: https://www.churchofjesuschrist.org/study/manual/gospel-topics/holy-ghost?lang=eng
The Power of the Holy Ghost is the ability of the Holy Ghost to influence, guide, or otherwise help God's children. This power can be experienced in many ways, but it is usually subtle rather than miraculous. The power of the Holy Ghost can be experienced by any one at any time in accordance with God's will, though we tend to notice and benefit from it more when we are trying to be good and do good. The power of the Holy Ghost in our lives is magnified when we are baptized and receive the Gift of the Holy Ghost (more on this below).
The Light of Christ is "the divine energy, power, or influence that proceeds from God through Christ that gives life and light to all things". The Light of Christ permeates the universe. It influences us to do good -- it is how we "just know" that some things are evil (like murder) and other things are good (like sharing). The Light of Christ can be hard to distinguish from the Power of the Holy Ghost, since both are influences that direct us toward God, and sometimes the Holy Ghost exercises his power through the Light of Christ. You can read more about the differences between them in my answer here: https://christianity.stackexchange.com/a/60384/32287
The Gift of the Holy Ghost is the promise that a person will have the power of the Holy Ghost as a constant force in their life as they strive to be worthy of it. This gift is given to those who are baptized by the proper priesthood authority. Baptism and receiving the gift of the Holy Ghost are two sides of a single coin: In being baptized, a person is forgiven of their sins and makes a covenant to live in such a way that the Holy Ghost can continuously guide them and help them improve. Baptism in water provides an initial cleansing, and then the continuous influence of the Holy Ghost enables us to be further cleansed and sanctified thereafter. It is this constant presence of the power of the Holy Ghost in our lives that allows us to achieve our full potential as children of God.
A revelation is simply a divine manifestation of any kind. It could be as dramatic as a heavenly visitation or as simple as a small feeling of peace. Obtaining an understanding of the truth of the Book of Mormon through prayer is a kind of revelation. Revelations are usually given through the power of the Holy Ghost.
Gifts of the Spirit are specific things that the Holy Ghost can give us in order to better accomplish God's will. Perhaps the most famous of these, though probably not the most common, is the gift of tongues (eg, being able to speak or understand a language that you have never learned). Such gifts are generally sacred rather than spectacular.
The Spirit of Revelation is one of the Gifts of the Spirit. It is available when the Holy Ghost "dwell[s] in your heart" and can therefore give you whatever knowledge you need in order to accomplish God's will. Those who have the Gift of the Holy Ghost and live worthily are natural recipients of the Spirit of Revelation. For more information, see https://www.churchofjesuschrist.org/study/general-conference/2011/04/the-spirit-of-revelation?lang=eng
Answers
Now to answer your questions! Yes, only those who are baptized and confirmed as members of the Church of Jesus Christ of Latter-day Saints can receive the Gift of the Holy Ghost. Once they have received the Gift, they must live worthy of it in order to maintain the constant companionship of the Holy Ghost.
Those who are not baptized can still experience some of the Power of the Holy Ghost, receive certain Revelations, and feel the influence of the Light of Christ, but they do not have the promise of constant companionship from the Holy Ghost.
What does that mean in practical terms? The constant companionship of the Holy Ghost is the state of being "born of the Spirit" which Christ mentions in John 3:5, saying "Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God". Thus, in order to achieve our full potential and eventually live with God in His kingdom, we must receive the Gift of the Holy Ghost and strive to keep our covenants. Those who do not do this will be unable to receive all that God has prepared for them in the eternities.
(Those who don't have the opportunity to be baptized in this life can still receive the Gift of the Holy Ghost through "baptisms for the dead" -- another discussion entirely)
Additionally, receiving the Gift of the Holy Ghost naturally grants greater access to the Gifts of the Spirit, such as the Spirit of Revelation and many other gifts.
I hope that answers your questions :)
