I might have considered rolling my eyes at this but as it's basically just another way of saying "the electoral roll used in general elections" I have no problem with it.
Gibraltar is a British Overseas Territory and joined the EEC (as it was) in 1974 when we did. They get to vote in our European elections, they are EU citizens but not all EU laws appyl . I suppose it is because of the strong link with the UK that they are included in the vote.
It's not exactly the same, I have found out that members of the House of Lords and Commonwealth citizens in Gibraltar can also vote but those groups are excluded from a General Election vote