What is the meaning of kissing the Gospel ?

“After having tasted and experienced in the Gospel how sweet the Lord is, how faultless His doctrine, how good and refreshing His consolations and promises, the heart of the priest overflows with happiness and joy, and he kisses the words of eternal life, in order to testify his profound reverence, his great and ardent love for them. This liturgical kiss, therefore, expresses what is contained in the verses of the Psalm: "More to be desired than gold and many precious stones are the words of the Lord; and sweeter than honey and the honeycomb. Greatly purified by fire is Thy word, O Lord, and Thy servant is exceedingly pleased therewith. Yes, I love Thy Commandments more than gold and precious stones ; they are the delight of my heart ; in my exile, they have become to me a canticle. I opened my mouth and I sighed; for I long for Thy Commandments."  "What the world values most is threefold: riches, whose principal representative and symbol is gold; beauty, represented by precious stones; and pleasure, symbolized by the honeycomb. Yet nothing of all that the earth can bestow is comparable to the joy and refreshment imparted by the word of God" (Reischl). The Gospel bestows that heavenly wisdom of which Solomon says : "I preferred her before kingdoms and thrones, and esteemed riches nothing in comparison of her. Neither did I compare unto her any precious stone: for all gold, in comparison to her, is as a little sand, and silver in respect to her shall be counted as clay. I loved her above health and beauty, and chose to have her instead of light; for her light cannot be put out" (Wisdom 7, 8 10).

Isaiah has a burning coal pressed to his tongue to purify it

If the Gospel is taken into the heart and preserved therein, with all that esteem and submission, love and joy, which the kissing of the book denotes, then is the Gospel also able "to blot out our sins." It is self-evident that no such power of effacing sin may be ascribed to the words of the Gospel, as is peculiar to the forms of the Sacraments of Baptism and Penance: they are only a kind of Sacramental in a more general sense and have, therefore, assuredly a great power of awakening and promoting that disposition of soul by which venial sins are effaced, or which prepares for and renders one worthy of receiving the Sacraments. The word of God, which is accompanied by the interior working of grace, exercises a redeeming, healing and sanctifying influence on man when he is properly disposed, by exciting faith, hope and charity, fear and contrition, conversion and amendment of life. It is not only a powerful means of clearing the soul of the excrescence of sin and imperfection, but it possesses, moreover, other beneficial effects besides. "Are not My words as a fire, saith the Lord, and as a hammer that breaketh the rock in pieces?" (Jer. 23, 29.) Yea, the words of the Lord are spirit and life: they are powerful, two-edged, penetrating. When Christ on the road to Emmaus "opened" the meaning to the two disciples of "the Scriptures, their hearts burned within them." The word of God has a marvelous power for enlightening the eyes, for imparting wisdom to the lowly and the humble, for rejoicing the heart and refreshing the soul. In like manner, may the living and quickening word of God, which abides forever, impart to us "salvation and protection," * may it purify, consecrate and sanctify our souls ever more and more. For "the Gospel is the power of God unto salvation to every one that belie veth" (Rom. i, i6)."

This exerpt was taken from The Holy Sacrifice of the Mass: Liturgically, Dogmatically and Ascetically Explained by Fr. Nicholas Gihr.  It is well worth the read!

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I would only further note that to clense Isaiah, God sent an angle to put a burning coal on his tongue.  So to, when the Gospel is kissed the priest is cleansed to go forth and proclaim the Gospel to the nations (The left side of the Altar represents the nations See this for more)