I made this table a few years back showing all classical references of Druids in both Britain and Gaul. Not much insight on the discussion, but it does shed some light on how classical authors looked at Druids.

Author	Dates	Text	Themes
Aristotle (attrib.)	384-332 B.C.	The Magikos, cited by Diogenes Laeterius (A.D. 225-250)Vitae 1.1	Druids are barbarian philosophers of the Celts and Galatai
Sotion (attrib.)	2nd cntury B.C.	Succession of Philosophers 23, cited by Diogenes Laertius (A.D. 225-250)Vitae 1.1	Druids are barbarian philosophers of the Celts and Galatai
Polyhistor	c. 105-35 B.C.	Cyril of Alexander (Archbishop A.D. 412-444)Contra Julianum 4	Polyhistor numbered the druids of Gaul among the barbarian philosophers
Cicero	106-43 B.C.	De Divinatione 1.90	Divination practiced by the druids, including Diviciacus, and Aeduan contemperary of Cicero.
Julius Caesar	100-44 B.C.	De Bello Gallico 6.13-14 De Bello Gallico 6.16.1-3 De Bello Gallico 6.18.1	Function of Druids Druids officiate at sacrifices Druidic tradition that the Gauls are descended from Dis.
Diodorus Siculus	c. 100 B.c. -	Biblioteke 5.31.2-5	Bards/druids/Manteis
Timagenes	80/75 B.C.-end 1st century B.C.	cited by Amm. Marc. 15.9.4-8 (Ammianus Marcellinus A.D. 330-c. 391)	Bards/druids/Euhagis
Strabo	63/3 B.C.-c. A.D. 21	Geographia 4.4.4	Bards/druids/Ouateis
Pliny	A.D. 23/4-79	Historia Naturalis 16.249 Historia Naturalis 20.103-4 Historia Naturalis 29.52 Historia Naturalis30.4	Druid mistletoe cutting ceremony. Druid's use of plants. The druid's egg, anguinum. A Vocontian put to death by Claudius for concealing one. Tiberius issued decree against druids, diviners, and physicians. The druids craft has crossed the Ocean.
Lucan	A.D. 39-65	Pharsalia 1.450-8	Druids inhabit groves. They profess knowledge of the gods/soul.
Mela	fl. c. A.D. 43	De Chorographia 3.2.18-19	Druidic knowledge of heavens. They instruct the nobles and meet secretly in caves or groves.
Tacitus	A.D. 56/7-120/123	Annals 14.30 Histories 4.54	Druids curse troops during Suetonius' attack on Mona. Druids prophecy end of Empire during Civilis revolt.
Dio Chrysostom	A.D. 40-112	Orationes 49	Celtic druids concern themselves with divination and wisdom.
Clement of Alexandria	c. A.D. 150-212	Stromata 6, 3.33.2	Druids are among the barbarian philosophers.
Suetonius	Early 2nd century A.D.	Claudius 25	Claudius suppressed the Druidic religion, which had been forbidden to Roman citizens by Augustus.
Hippolytus	c. A.D. 170-236	Philosophumena 1.25	Celtic druids as 'Pythagorean' philosophers, seers, and magicians.
Diogenes Laertius	fl. A.D. 225-250	Vitae 1.1 (see Aristotle, above) Vitae 1.5	See above. Druids pronounce by means of riddles and dark sayings.
Lampridius	3rd century A.D.	Scriptores Historiae Augustae Alexander Severus 60.6	A female Gallic druid prophesied the defeat of Severus.
Vopiscus	3rd century A.D.	Scriptores Historiae Augustae Aurelian 44.4 Scriptores Historiae Augustae Numeranius 14	Aureleian asked female druids in Gaul if the imperial power would remain with his descendents. A female druid prophesied Diocletian would be Emperor.
Ausonius	4th century A.D.	Commem. Prof.4.7.10 Commem. Prof. 10.22.30	Attius Patera descended from the druids. Phoebicus descended from the druids of Bordeaux.