Is it a good ideal to back Libra with Gold?
Yes, it is.
In case of a global currency crises, involving all national fiat currencies, Libra won't be a refuge, as it's backed by fiat currencies.
Libra is backed also by Government bonds, but a crises in national fiat currencies means a crises in Govs too, so being backed by Gov debt won't help.
In case of a global currency crises, the refuge will be some hard asset, i.e. commodities.
Historically, the commodity that best suited to give stability and confidence to a currency was gold.