I don't think it's unreasonable. Government spending is in a sense an arbitrary number. About half of the budget is money that comes in one door and out the other; they're wealth transfer programs rather than actual "spending". Such programs could, if we wished, be many times GDP. It's a bit like basing a bank's value based on the size of its deposits, even though every dollar on deposit is also a dollar that they owe. I'm not taking a stand on the programs one way or another, simply pointing out that the size of the budget isn't an easy number to interpret for comparison purposes.
The GDP, on the other hand, more or less corresponds to something real: how much the nation produces. There are numerous ways in which the calculation is flawed, and that number too is most effective only when compared to historical data. But in this case, it's a not-completely-insane way to say, "This is how much the nation makes, and this is what fraction we spend on protecting that earning capacity via intelligence services."
It would also be meaningful to compare to real government spending (as opposed to the government's supervision of transfer payments). But that number is roughly proportional to GDP, since it effectively takes a fraction of GDP in taxes. So it's another way of saying the same thing.
You can certainly dispute whether that amount is still too much, or whether the amount is being spent well in pursuit of that protecting-the-rest-of-our-earnings goal. But I don't think it's meaningless to compare the two numbers.