  Most major scientific results produced by ground-based gamma-ray telescopes
in the last 30 years have been obtained by expert members of the collaborations
operating these instruments. This is due to the proprietary data and software
policies adopted by these collaborations. However, the advent of the next
generation of telescopes and their operation as observatories open to the
astronomical community, along with a generally increasing demand for open
science, confront gamma-ray astronomers with the challenge of sharing their
data and analysis tools. As a consequence, in the last few years, the
development of open-source science tools has progressed in parallel with the
endeavour to define a standardised data format for astronomical gamma-ray data.
The latter constitutes the main topic of this review. Common data
specifications provide equally important benefits to the current and future
generation of gamma-ray instruments: they allow the data from different
instruments, including legacy data from decommissioned telescopes, to be easily
combined and analysed within the same software framework. In addition,
standardised data accessible to the public, and analysable with open-source
software, grant fully-reproducible results. In this article we provide an
overview of the evolution of the data format for gamma-ray astronomical data,
focusing on its progression from private and diverse specifications to
prototypical open and standardised ones. The latter have already been
successfully employed in a number of publications paving the way to the
analysis of data from the next generation of gamma-ray instruments, and to an
open and reproducible way of conducting gamma-ray astronomy.
