I don't think a variant of the Gilded armour should be obtainable in SGS the same way existing retro armour can be bought there. Gilded is in between the other Rune trimmed and Third-age rarity. Getting a
specific
piece of Gilded is as rare as getting
any
3rd age piece on Hard clues.
Imagine if 3rd age gets graphically updated and people suggest the old armour to become available in SGS. This means people no longer have to pay game gold and instead can resort to mtx for an older version of the same armour.
Instead of buying old gilded directly via SGS, maybe you can buy some sort of new 'retro keepsake key' that allows you to turn certain in game equipment into their respective retro override variants, so that the updated equipment (in this case gilded) doesn't crash because people buy a slightly different variant of them with RuneCoins instead.
Feel free to take a look at my suggestions:
200m xp prestige