Here's the reply from NSF:
"NSF International has not tested any gutter products for contact with drinking water and could not advise if they might meet potable water standards or be suitable for use in this type of garden. As a suggestion, I would encourage you to contact your nearest county extension office and request to speak with a master gardener about what materials might be suitable for an above ground garden.

Best regards,
Cheryl

Cheryl Luptowski
NSF Consumer Information Officer
info@nsf.org"
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I'd be content to wash the PVC carefully, then line it with food safe plastic film, or to line it with corrugated cardboard, and I think I'd only use the pipe/gutter containers to start plants prior to planting them in the garden.

Alternatively, what other, more stable materials might work in a similar manner?
Your mileage may vary.