  Global cosmic strings are predicted in many motivated extensions to the
Standard Model of particle physics, with close connections to axion dark matter
physics. Recent studies suggest that, although subdominant relative to
Goldstone emission, gravitational wave (GW) signals from global strings can be
detectable with current and planned GW detectors such as LIGO, LISA,
DECIGO/BBO, ET/CE and AEDGE/AION, as well as pulsar timing arrays such as PPTA,
NANOGrav and SKA. This work is an extensive, updated study on GWs from a global
cosmic string network, taking into account of the most recent developments
related to the subject. The main analysis is based on the analytical
Velocity-dependent One-Scale (VOS) model calibrated with recent simulation
results, which provides a generic protocol for such calculations with details
given. We also demonstrate how the GW signal can be influenced with variations
to the baseline model: this includes considering the uncertainties of model
parameters and the potential deviation from the conventional VOS model
prediction (i.e. the scaling behavior) as suggested by some of the recent
simulation results. Furthermore, we investigated in detail the effect of a
non-standard cosmology (e.g. early matter domination or kination) or new
particle species on the GW signals from global strings. We demonstrate that the
frequency spectrum of GW background from global cosmic strings can be used to
probe the cosmic history prior to the Big Bang nucleosynthesis (BBN) (i.e. the
primordial dark age) up to a temperature of $T\sim 10^8$ GeV.
