It looks like increased gamma activity is pronounced simply for long-term meditation practitioners.
Long-term Vipassana meditators sat in meditation vs. a control rest
(mind-wandering) state for 21 min in a counterbalanced design with
spontaneous EEG recorded. Meditation state dynamics were measured with
spectral decomposition of the last 6 min of the eyes-closed silent
meditation compared to control state. Meditation was associated with a
decrease in frontal delta (1–4 Hz) power, especially pronounced in
those participants not reporting drowsiness during meditation.
Relative increase in frontal theta (4–8 Hz) power was observed during
meditation, as well as significantly increased parieto-occipital gamma
(35–45 Hz) power, but no other state effects were found for the theta
(4–8 Hz), alpha (8–12 Hz), or beta (12–25 Hz) bands. Alpha power was
sensitive to condition order, and more experienced meditators
exhibited no tendency toward enhanced alpha during meditation relative
to the control task. All participants tended to exhibit decreased
alpha in association with reported drowsiness. Cross-experimental
session occipital gamma power was the greatest in meditators with a
daily practice of 10+ years, and the meditation-related gamma power
increase was similarly the strongest in such advanced practitioners.
The findings suggest that long-term Vipassana meditation contributes
to increased occipital gamma power related to long-term meditational
expertise and enhanced sensory awareness.
Early studies of meditators implicated alpha (8–12 Hz) power increases
as both a state and trait effect of Yogic, Zen, and Transcendental
Meditation practice. Later studies have failed to replicate the early
findings of increased alpha in advanced practitioners but have
reported increased alpha coherence, especially in assays of TM
practitioners (Gaylord et al. 1989; Travis 1991; Travis and Pearson
1999; Travis et al. 2002), theta (4–8 Hz) power, especially in the
assays of concentrative/focused attention practitioners (Aftanas and
Golocheikine 2001; Baijal and Srinivasan 2009; Hebert and Lehmann
1977; Pan et al. 1994), or gamma effects (Lehmann et al. 2001; Lutz et
Cahn, B. Rael, Arnaud Delorme, and John Polich. "Occipital gamma activation during Vipassana meditation." Cognitive processing 11.1 (2010): 39-56. doi: 10.1007/s10339-009-0352-1
Whether one can accelerate the gamma power with a particular meditation training technique is another question, however.
Activities likely to trigger gamma power
Even if particular subtypes of mindfuless meditation techniques promoting gamma power in novices may not be known yet, we can conjecture the characteristics thereof based on other activities that trigger gamma activity in the prefrontal cortex. From the comments, Angela Richardson writes
Noise, flashing lights, movement and high-contrast or striped patterns
can all increase gamma activity. In some people the resulting increase
in gamma activity is so large that it causes seizures or migraines. If
you want a meditation exercise to increase your gamma power then
imagine loud music, strobe lights and brightly coloured rapidly moving
shapes and patterns.
However, the corresponding gamma waves are primarily observed in the visual cortex. See
Adjamian, Peyman, et al. "Induced visual illusions and gamma oscillations in human primary visual cortex." European Journal of Neuroscience 20.2 (2004): 587-592. https://doi.org/10.1111/j.1460-9568.2004.03495.x
Azizi et al. note that gamma waves also get activated during an n-back exercise, which requires active focus and working memory utilization:
during cross-modal sensory processing (perception that combines 2
different senses such as sound and sight). Also, it is shown during
short-term memory matching of recognized objects, sounds, or tactile
Roohi-Azizi, Mahtab, et al. "Changes of the brain’s bioelectrical activity in cognition, consciousness, and some mental disorders." Medical journal of the Islamic Republic of Iran 31 (2017): 53. https://doi.org/10.14196/mjiri.31.53
A common theme in the gamma activity literature seems to be a mental state that is awake and alert, with a deep and selective focus/attention. It has also been observed in the phenomenon of "getting it right" and feature extraction, as Phillip Gilley demonstrates in this beautiful visualization:
A mental state that sufficiently mirrors these characteristics during meditation may possibly lead to the desired gamma activity.