[Answer from high-church Anglican/Catholic perspective.]
The deacon is not censed — or should not be censed — at the reading of the gospel. The thurifer should hand the thurible to the deacon, who censes the book. The thurifer generally remains while the gospel is read, and indeed the gospel may end up proclaimed from within the cloud.
- a ceremonially purifying agent
- a symbol of honour (incense was given to the Christ-child and used in Temple worship)
- a symbol of prayer rising (cf Ps 141:2)
- a pleasant smell (the smell of heaven, as opposed to sulphur, the smell of The Other Place)
The deacon is not censed at this point because he is not the Word, he is merely the agent. The point of censing the gospel is to show it honour; it is the Word proclaimed.
As to “official sources” (from the comments), that’s surprisingly difficult. The modern documents of the Roman Church such as the General Instruction of the Roman Missal don’t lay down the ceremonial; it’s all tradition.
The English Missal is just about the most authoritative extant English source for high ceremonial, is available and more accessible. It was published in 1921, and probably reflects what was laid down at the Council of Trent, which carried out the last major revision of Catholic worship.
Thurifer While the responsory is in singing the Deacon shall place the Book of the Gospels on the Altar: and the Priest, the Deacon ministering the incense-boat, shall place incense in the censer. Then shall the Deacon say the prayer: Cleanse my heart, and taking the book from the Altar ask a blessing of the priest. Afterward, the Subdeacon holding the book between two Acolytes with lighted candles, the Deacon shall sign and cense the book, and sing the gospel in the customary manner, whereafter the Subdeacon shall bear the book to be kissed by the priest, who is also censed by the Deacon.
The book was (is) censed before to show honour to the proclaimed Word; the celebrant was censed afterwards because he is acting in Persona Christi, which I can see might raise all sorts of other questions.
While this level of ceremonial can be found these days if you look hard enough, most celebrations nowadays are simpler; some actions have been removed. In particular, the book of the gospels is kissed by the person who reads it unless the celebrant is a bishop; and the celebrant is not censed after the reading. The Institutio Generalis Missalis Romani published in 1969 lays down what should happen now, in the absence of specific directions in the 2010 version.
131 Dum Alleluia vel alter cantus profertur, si adhibetur incensum, ad impositionem thuris secerdoti ministrat, deinde, ante sacerdotem inclinatus, benedictionem petit, submissa voce dicens, Iube, domne, benedicere. Sacerdos eum benedicit, dicens: Dominus sit in corde tuo, etc. Diaconus respondet: Amen. Deinde librum Evangeliorum, si est in altari, sumit et ad ambonem pergit, ministris, si adsunt, cum candelabris et incenso pro opportunitate eum praecedentibus. Ibi populum salutat, librum incensat et proclamat Evangelium. Quo finito, librum osculo veneratur, secreto dicens, Per evangelica dicta etc., et ad celebrantem redit. Si vero homilia non fit neque symbolum dictur, potest manere in ambone pro oratione universali, recedentibus ministris.
Ceremonial is never added, it's only simplified. Between 1921 and 1969, the ceremonial changed so that the book is no longer routinely processed to be kissed and the celebrant is no longer censed. The deacon himself has never been censed at the gospel.