He doesn't need to set markings up around the area to dodge Hirudora. He's fast enough to simply toss a Kunai away from the area while Hirudora is coming at him to evade it. Gai can't kill Tobirama before he uses Hiraishin to evade his attacks and to do that he'll need to throw a marking. So eventually markings get placed and then Tobirama learns more about Gai's speed and then lays more markings and uses clones to try and take him out.
And not even Minato spreads Kunai around as a normal strategy. He only did it once against Ay due to his knowledge on Ay's speed. Against Obito, in the War Arc are his other 2 fights and he never spread markings like that.
Without intel Tobirama won't know the mechanics of Hirudora or what it will do. There's no way he's escaping without some type of special set up, I can't post scans because I'm on a phone but the turtle island explosion was much to large for that tactic. Especially if Gai starts off with an initial blitz like he did against Madara, which Tobirama can't evade but can react to.
Which is why intel is such a big deal here
With intel he can start off with bunshin reducing Gai's chance of hitting the real Tobirama, and spread Kunai before Gai attacks. Leaving Gai with few options since he can't do Hirudora more than maybe twice, three max, and he's caput. Not having to worry being able to escape lets him play offensively to, or use bait and switch tactics.
For me this basically comes down to wether Tobirama knows the capabilities of a Gates user. If he does he wins, if not he loses.
*Edit: Minato did spread Kunai during the War arc, just not blantantly and excessively. He put them randomly in the ocean, threw 4 for the Kage, and three others here and there.
