Several early examples are variations on the pattern that do not strictly use the "eighth note followed by two sixteenth notes", but nonetheless have been identified as gallops.
^Popoff 2018b, p. 67: "prototype of the heavy metal gallop"
^Hal Leonard 2011, eBook: "key heavy metal rhythm commonly called the gallop"
^Encabo 2015, p. 35: "The opening guitar riff is based on the so called gallop rhythm"
^Popoff 2011, p. 247: "sublime chords set to a soft gallop"
^Popoff 2018a, p. 32: "a classic Iron Maiden gallop"
^ abChlasciak 2018: "utilizes a classic metal 'gallop' rhythm, which is an eighth note followed by two 16ths"
^ abcPillsbury 2013, eBook: "Tightly palm-muted ostanati are also heard in other Metallica songs, ranging from the straightforward gallop in 'Motorbreath' from Kill 'Em All to the faster, more complex figures used in the verses of 'Disposable Heroes' and 'Damage, Inc.' from Master of Puppets.
^ abRevill 2017: "['Holy Diver' uses] a much slower and simpler version of a gallop rhythm is in the verse riff"; "This version [used in 'Raining Blood'] is commonly known as the Reverse Gallop"
Sources
Chappell, Jon; Phillips, Mark; Serna, Desi (2014). Guitar All-In-One For Dummies. John Wiley & Sons. ISBN978-1118872109.
Stang, Aaron; Purse, Bill (2014). Sound Innovations for Guitar, Teacher Edition Book 2: A Revolutionary Guitar Method for Individual or Class Instruction. Alfred Music. ISBN978-1470633172.