Risk 1: Impersonation. ie, someone sets up hmrc.gov.gb and tells people "pay your taxes here" and scams a bunch of people.
Risk 2: Economic loss. People selling .gb domains might take away from .uk revenue.
Risk 3: Political fallout. Perhaps Chagos Islanders somehow get control .gb and demand that the UK give up .io in exchange. There's plenty of geopolitics that could get brought up.
Risk 4: opportunity cost. Perhaps later some major use for .gb arises, but by then it's too late.
These are all pretty small risks, but this is the kind of thing they may have in mind.
Risk 2: Economic loss. People selling .gb domains might take away from .uk revenue.
Risk 3: Political fallout. Perhaps Chagos Islanders somehow get control .gb and demand that the UK give up .io in exchange. There's plenty of geopolitics that could get brought up.
Risk 4: opportunity cost. Perhaps later some major use for .gb arises, but by then it's too late.
These are all pretty small risks, but this is the kind of thing they may have in mind.