In British cryptic crosswords, GG is often used as an abbreviation for 'horse'.
FT 13588 (Orense): Move from side to side in horse riding trick (6) WIGGLE
GG (horse) riding i.e. being carried by WILE (trick)
GG or gee-gee is a word for horse used by children or in colloquial speech in UK. The internet provides different explanations for its etymology. Wikipedia says that the term "Gee-Gee" is taken from horse-racing where a Gee-Gee is the first horse out of the starting gate. Other sources say that GG is short for the command given to horses to go: "gee up". I'm not sure which is the actual origin of the word, comments that can shed light on this are most welcome.
In any case, for solving cryptic clues, it is sufficient to know that horse = GG and that the term is mostly used by children.
Have a go at these clues that use horse = GG:
Independent 7746 (Anax): Pushes horse in front over (4, 2)
Guardian 24635 (Enigmatist): Less diminutive horse in nursery found in the final frame (6)
Guardian 25476 (Boatman): Startling horse eaten by naughty goblin (8)
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