Both he and Alex Salmond are 59, both started out at the Scottish Office in the late 1970s, both built careers around the politics of devolution and independence, and both claim to know the result on September 18.
But there the similarities end.
While Salmond sprang onto the public stage, Gallagher remained in the wings of the civil service, advising a series of Scottish secretaries and prime ministers.
He also ran the Scottish Prison Service for five years, and was secretary to the 2008 Calman Commission on greater devolution. "My natural place is in the back room rather than the front room," as he puts it.
After retiring in 2010, he seemed destined for sleepy academe as a research fellow at Oxford.
The referendum changed all that.
A dogged indysceptic - he describes the case for the Union as "unanswerable" - he felt compelled to step up and become involved.
In January, he became a key adviser to the Better Together campaign, honing its messages on the economy, currency and the "positive case" for the Union, as well as chipping in to Alistair Darling's debate prep.
"My contribution has been giving some shape to the argument of why you're better off within the United Kingdom ... explaining the risks, but also why you should make a genuinely positive choice to stay in the UK rather than merely reject an uncertain future."
So what's his 10-second doorstep pitch?
"You get the best of both worlds. You get the benefits of being a small country with control over your own domestic affairs, the ability to make choices about public services and, increasingly, taxation. But also the benefits of being a large country, the security of being part of a larger economy, a country that's economically and politically powerful."
He admits Better Together is sometimes seen as "too negative", but adds: "What people call negative campaigning - saying what's wrong with the other side's argument - is just another way of making your own positive argument.
"The mirror image of those [risks we highlight] are the strengths of staying in the UK.
"I've been really struck by the extent that the independence case, rather sadly, is becoming a negative case. It's saying, 'Oh, there's all these things wrong with the UK.'"
Oh, come on, Better Together have been negative from the first minute of this campaign, no? He bridles and says that's not fair criticism.
"Here are people, the Scottish Nationalists, the Yes campaign, saying we should deconstruct the state we're in and create a new one.
"It's only commonsense to say, 'What are the risks?' You can talk about potential benefits, but all these things come with risks. Our job, among things, is to point out those risks."
He has some criticism for his own side, too. Westminster "missed a trick" and "undersold" the 2012 Scotland Act which gives Holyrood new devolved tax powers, he says, leaving the public largely in the dark about the changes already in train in the event of a No vote.
But basically he's a massive UK fanboy. "The UK said to Scots, 'It's entirely up to you: you can stay or leave.' If this referendum was really about whether the Scottish people are sovereign, the very fact of having it says they are. The UK acknowledged that almost without a hiccup. It's an extraordinarily grown-up approach to statehood."
Unlike the hopelessly immature SNP, he says. "I'm totally fed up with them ... it's about time they started being grown-ups. I don't agree with it, but there's an honourable argument to make for independence.
"The problem with the Scottish Government is they say it doesn't carry any risks.
"Nothing will go wrong, everything will be better, taxes will go down, spending will go up, there'll be money in the bank. That isn't true," he goes on. "They know their basic argument isn't where the people are, so they try and tell them something else. It would be better for their self-respect and their long-term project to make a more honourable argument."
What's the referendum result going to be?
"I'm as certain as I can be that people are going to reject independence and they're right to do so. Public opinion has been stable on this for years, if not decades."
Gallagher offers an equally striking take on the indyref row of the week, Salmond's refusal to name a Plan B if there's no currency union.
Sharing the pound wouldn't just be bad for the UK, it would be bad for Scotland, he says.
"Currency union is not just about using the same notes and coins, it's about having a single macroeconomic arrangement. You'd be tying yourself to a macroeconomic system managed for somebody else's economy and you couldn't move public spending back and forth. It's very hard to see why, except for presentational reasons at the beginning, that a Scottish Government would agree to it."
So what is the best Plan B? "The long-term right answer is your own currency. Absolutely no doubt about that. How you get there, God knows. It's not for me to make that one up."