The GBI is a guaranteed income stream, right? So, can I sell it? Can I put it up as a collateral for a loan?
It's up to the willingness of the country giving out the loan to regulate whether it wants to enforce those loans. Many countries have a legal system where you can't collect money from people who need that money to live.
whether it wants to enforce those loans
Generally speaking yes, it does. What particular kind of contracts do you think a country wouldn't want to enforce in this context? As to subsistence-level income not being collectable, see my answer to TheAncientGeek.
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