Below is a list of Gaulish words found in Dioscorides, De Materia Medica (trans. Osbaldeston and Wood, 2000). De Materia Medica is a compendium of plants, animals and minerals and their medicinal uses, compiled by Pedanius Dioscorides around 60 AD. He identifies each plant by its Greek name, but often gives a number of alternate names, usually in Greek or Latin, but sometimes in Egyptian, Syrian, Dacian, Gaulish, etc.
Each Gaulish word is followed by the book and chapter number where it appears, the chapter heading (a Greek name in Latin all-caps), the Linnaean or other taxonomical name as suggested by Osbaldeston and Wood, and a common English name. I have only listed the words that Dioscorides specifically credits to the Gauls; there could be other Gaulish words attached to phrases like 'some others call it'. Because Dioscorides credits the Latin phrase piper apum (bees' pepper) to the Gauls I have included it in the list.
albolon 3.36 GLECHON. Mentha pulegium. Pennyroyal.
baccar 1.9 ASARON. Asarum euopaeum. Hazelwort.
beliucandas 4.115 MURIOPHULLON. Myriophyllum spicatum. Water milfoil.
bilinuntiam 4.69 UOSKUAMOS MELAS, UOSKUAMOS LEUKOS, UOSKUAMOS MELOIDES. Hyoscyamus niger. Henbane.
corna 2.208 ARGEMONE. Papaver argemone. Pale poppy.
ducone 4.175 CHAMAIAKTE. Sambucus ebulus. Dwarf elder.
gelasonen 3.132 GNAPHALION. Filago germanica. Cudweed.
iumbarum 4.16 LEIMONION. Pyrola rotundifolia. Wintergreen.
jupisellusum 1.103 ARKEOTHOS MEGALE, ARKEOTHOS MIKRA. Iuniperus minor. Lesser juniper.
laginum 4.150 ELLEBOROS. Veratrum album. Hellebore.
merisimorion 3.118 MELISSOPHULLON. Melittis mellisophyllum. Honey balm.
ousubim 4.148 DAPHNOIDES. Daphne mezereum. Laurel.
ousubim 4-149 CHAMAIDAPHNE. Prunus laurocerasus. Dwarf laurel.
pempedula 4.42 PENTAPHULLON. Potentilla alba. Cinquefoil.
piper apum 1.2 AKORON. Iris pseudacorus. Yellow flag.
ponem 3.127 ARTEMISIA MONOKLONOS, ARTEMISIA MONOKLONOS ETERA. Artemisia vulgaris. Mugwort.
sapana 2.209 ANAGALLIS, KORKOROS. Anagallis arvensis. Scarlet pimpernel.
scobie 4.174 AKTE. Sambucus nigra. Elder tree.
scubulum 4.71 STRUCHNOS KEPAIOS. Solanum melongena. Eggplant.
sistrameor 3.82 IPPOMARATHRON. Foeniculum vulgare. Wild fennel.
subites 2.210 KISSOS. Hedera helix. Common ivy.
tarbidolopion 2.153 ARNOGLOSSON, ARNOGLOSSON MIKRON. Plantago major. Greater plantain.
theximon 3.6 ARISTOLOCHIA KLEMATITIS. Aristolochia clematitis. Apple of earth.
thona 2.211 CHELIDONION. Chelidonium majus. Greater celandine.
Source: Dioscorides, Materia Medica.
Further reading: Dottin, La langue gauloise.