Which (and I know you know this ) is exactly the point of Gladiator’s weakness. He’s SUPPOSED to be a direct Superman analogue (even down to the Kryptonian / Strontian thing of naming the race after a chemical element), and his radiation weakness is specifically supposed to directly mimic Superman’s reaction to kryptonite radiation.
I’ve not weighed in on this so far, but my take is this:
1) The radiation weakness is not a vulnerability like Diana’s compartmentalised durability because it doesn’t just harm him, it specifically makes him weaker against everything else as well. Diana getting hit by bullets doesn’t make her more punchable. Gladiator’s thing is like a combination of some really vicious virus (I am completely blanking on naming a specific nasty virus at the moment for some reason) draining him of his strength, and Space AIDS making him weak against anything else. It knocks the Gladiator out of him.
2) I’ve never thought about the impact of specific weaknesses in a composite before, because it’s kind of a niche topic, but it makes perfect sense that it would have an impact.
3) Here’s my take on how it would work:
a) Gladiathor gets hit by the radiation.
b) The Thor part of Gladiathor is not vulnerable to the radiation, but basically gets the Gladiator knocked out of him, so he’s more or less now just Purple-Thor-with-Mohawk-Mullet (seriously, take a moment to think about what an abomination Gladiathor’s hair would be like *shudder*). After all, a composite uses the strongest of its constituent parts in any particular situation. When exposed to the radiation, the Gladiator is knocked out of Gladiathor, so without his Gladness, Thor is the strongest part in pretty much every circumstance, so this is just Bad Hair Thor vs Surfer.
c) Surfer, with his shiny head and classic bald man complex, might actually have a specific weakness to people who are blessed with actually having hair but then abuse that privilege the way Gladiathor has with the Mohawk-Mullet. However, it doesn’t do enough psychological damage to take away Surfer’s speed advantage and his eleventy-twelve different ways of murking Thor and taking away his offensive coiffuring.