It’s been a while and I wasn’t doing it all from script but I’m pretty sure you should be able to do this.

The fields are on the item sublist itself (as opposed to a subrecord) and you can find them in the record browser:

giftcertfrom
giftcertmessage
giftcertrecipientemail
giftcertrecipientname

Plus there’s giftcertnumber which doesn’t seem to be in the records browser.

So you set them like:

.setSublistValue({
    sublistId: 'item',
    fieldId: 'giftcertnumber',
    line: i,
    value: ...
});