This conference proceedings is a write-up of the Gamma-ray Indirect rapporteur talk given at the 37$^{\rm{th}}$ International Cosmic Ray Conference (ICRC 2021). In contrast to previous ICRCs, this years edition was held in a fully virtual format, with dedicated discussion sessions organised around specific scientific themes. Many of these topics span the two categories of Gamma-ray Indirect (GAI) and Gamma-ray Direct (GAD), observations of gamma-rays by ground-based and space-based facilities respectively. To cover this organisation by topic in a coherent manner, this GAI rapporteur contribution focuses predominantly (but not exclusively) on Galactic gamma-ray astronomy, whereas the GAD rapporteur contribution focuses predominantly (but not exclusively) on Extra-galactic gamma-ray astronomy.
In recent years, the field has seen enormous progress in both theory and observation, particularly in identifying PeVatrons (accelerators of Cosmic Rays to PeV energies), studies of particle escape from the accelerator, and detection of gamma-ray transients, especially gamma-ray bursts.