He has been described as "a pivotal figure in the development of archaeobotany at the Institute of Archaeology at University College London, [who] through his research, publications and teaching had a major influence on the field worldwide.

His first excavation (1969–70) was the Pre-Pottery Neolithic B site of Can Hasan III on the Konya Plain of central Turkey, where large-scale flotation recovered early domesticated plants, including rye.

This was a pioneering multi-disciplinary excavation of four sites around the village of Aşvan, led by David French and involving a wide range of specialists, including those from anthropology, geography, zoology and botany.

[13] Hillman and Harris formed an effective team, most notably in fieldwork in Syria and Turkmenistan, and in organising the conference session at the Southampton World Archaeological Congress of 1986 that led to the publication of Foraging and farming: the evolution of plant exploitation in 1989.

Something of the atmosphere of that period is conveyed in the introduction to Hillman's Festschrift volume: "Anyone exiting the third floor lift in the Institute of Archaeology in the 1980s and 1990s would have been confronted with the sight of Gordon's office, at times shared with up to three other colleagues and crammed full of books, cereal sheaves and reaping hooks, with at least 1-3 students and visiting colleagues, taking full advantage of his good nature, deep knowledge and awful coffee.

[18] Further fieldwork by Glynis Jones in Greece put Hillman's results onto a firmly quantified basis, and this mode of interpretation of plant remains - in terms of crop processing stages such as winnowing and sieving - is now a standard component of archaeobotany, particularly in the Old World.

[18] Many of Hillman's students carried out ethnoarchaeological work, for example Sarah Mason and Mark Nesbitt in Turkey,[19] Catherine D'Andrea and Ann Butler in Ethiopia,[20] and Leonor Peña-Chocarro and Lydia Zapata in Spain and Morocco.

[14] The results of his botanical fieldwork were most fully explored for the site of Abu Hureyra, for which his 1996 paper and 2000 book make extensive use of current day plant distribution to model the availability of wild cereals and other foodstuffs in the Epipalaeolithic period.

[24][11] Hillman also carried out experimental harvesting of wild cereals, leading to highly influential work with the geneticist Stuart Davies on modelling the potential speed of wheat domestication.

[25] They concluded that selective pressures meant that morphological domestication in the form of loss rachis fragility could occur within 200 generations, thus 200 years for this annual crop.

Difficulty in identifying the fragmented plant remains characteristic of early sites led Hillman to build an excellent seed reference collection.

[27] Working with his students, Hillman explored a wide range of identification techniques including tuber and wood anatomy,[28] infra-red spectroscopy and other forms of chemical analysis.

However, following detailed ecological modelling, Hillman and Moore proposed instead that the wild cereals had been under cultivation, probably in response to desiccation caused by the Younger Dryas climate event at about 12,900 to c. 11,700 years BP.

[36] Connolly and Colledge propose instead that the observed shift in plant consumption to less desirable foodstuffs at Abu Hureyra 1 simply reflects the greater scarcity of wild cereals under the effects of the cooler, drier Younger Dryas climate.

In last 20 years, post-retirement, reconstructing the potential foraging diet of pre-agrarian Britain became Hillman's main project, in part carried out with Ray Mears, the bushcraft instructor.

[41] Incorporating copious experiments in processing to remove toxicity and improve taste, and extensive use of ethnographic and archaeobotanical data, the resulting plant profiles are currently being edited and released by colleagues at UCL.

[39] He also collaborated with food scientists Tony Leeds and Peter Ellis at King's College London, leading to nutritional analyses of acorns[19] and sea club-rush (Bolboschoenus maritimus) tubers.