what does that = to granular sugar
Caster sugar is finer than granulated sugar. It's supposed to dissolve more quickly, but in a cake, I think a 1:1 substitution should be fine.
275 grams of granulated sugar will have the same sweetness as 275 grams caster sugar, which is like superfine sugar. The finer sugar will make a slightly better texture, and you can blitz granulated sugar in the food processor to mimic caster sugar if you want to take the extra step.
Since the measurement is given by weight, you simply substitute 275 gm of granular sugar. Had it been given in volume (cups) then there would have been a slight disrepancy. If you wish to stay true to the recipe, simply blitz regular granulated sugar in a food processor for a few seconds and you will have caster sugar.