We shall find as we proceed that many of his arguments are the same as those of the earlier Apologists, Aristides, Justin Martyr, Tatian, Clement of Alexandria, and Origen; that he constantly borrows long passages from their writings, including the same quotations from Greek authors, reproduced word for word with due acknowledgement. Those earlier authors had in fact adopted the very same method which Eusebius announced as distinctive of his own work. The quotations thus borrowed are however few in comparison with the great multitude gathered by Eusebius himself from all parts of the Greek literature of a thousand years, from works both known and unknown of poets, historians, and philosophers.

...The list of writers quoted or referred to is astonishing for its length (see Fabric. Bibl. Graec. vii. 346). Some of these are known to us, even by name, only through Eusebius, and of several others he has preserved large portions which are not otherwise extant.

Galen is related to heresy in the Church History [see 5.28.14, where along with Aristotle and Theophrastus he is said to be held in high esteem by some heretics who tamper with scriptures] and, perhaps for this reason, there is no trace of him in the Gospel Preparation, for which Eusebius and his assistants ransacked the church library at Caesarea.[n.96 K.Mras, Eusebius Werke 8. Die Praeparatio Evangelica (Berlin: Akademie, 1954-1956), thought Galen's Exhortation was related to a quotation in Gospel Preparation 5.27.8, 33.8, but the quotation (from Oenomaus of Gadara) is not what Galen quoted.] Presumably none of Galen's works was included in the collection, any more than works by Aristotle.

[The following are mentioned in Gifford's summary of contents]
Abydenus, author of the Assyrian History, the
Alexander of Aphrodisias,
Alexander Polyhistor (cites Eupolemus, Artapanus, Molon, a certain Philo, and Demetrius),
Amelius,
Areius Didymus (Stoic bk 15.15)
Aristippus,
Aristocles the Peripatetic (bk 14.17-21, 15.)
Aristotle,
Atticus the Platonist (bk 15.4-9)
Choerilus the poet,
Clearchus the Peripatetic,
Diodorus Siculus (cites Euemerus),
Diogenianus,
Dionysius of Halicarnassus,
Epicurus,
Euripides,
Hecataeus of Abdera,
Longinus (bk 15.21)
Metrodorus, and
Numenius (bk 14)
Oenomaus,
Pindar,
Plato,
Plotinus, 
Plutarch,
Porphyry,
Proclus,
Protagoras,
Pyrro and the Pyrrhonists/Skeptics (bk 14)
Severus (Platonist;  bk 13.16)
Sibyl, and others,
Xenophon,

Africanus,
Aristobulus ("the Peripatetic" bk 13.12)
Bardesanes the Syrian,
Clement of Alexandria,
Dionysius of Alexandria (bk14.23-27)
Ezekiel (Jewish dramatist)
Josephus,
Methodius,
Origen,
Philo,
a Jew named Philo,
Philo Biblios (via Porphyry),
Tatian,
Theodotus (epic poem on Jews),