Você pode usar date -u
( tempo universal ) que é equivalente a GMT.
Citando date
manual:
‘-u’ ‘--utc’ ‘--universal’
Use Universal Time by operating as if the ‘TZ’ environment variable were set to the string ‘UTC0’. UTC stands for Coordinated Universal Time, established in 1960. Universal Time is often called “Greenwich Mean Time” (GMT) for historical reasons. Typically, systems ignore leap seconds and thus implement an approximation to UTC rather than true UTC.