Tor, this is indeed a straw-man argument. It's not as complicated as you claim.
Gregory of Tours is a primary source for things that he knew from his own experience. He's a secondary source for things he got from other people. Indeed. That's the definition of primary and secondary sources. Not hard.
Gregory of Tours is probably a reliable source for things near his own time. There is no way to know whether he is a reliable source for things beyond his own time. That shouldn't be hard to understand.
In other words, there is not a single rule that you can apply to everything he wrote. He's probably not absolutely right about everything and he's probably not absolutely wrong about everything. That shouldn't be hard to understand.
You must ask yourself -- do you really believe that Gregory's account of the creation of the world has the same credibility as his account of events that took place while he was bishop of Tours? If you are not prepared to make that leap, you have already begun to understand the idea that not everything he wrote has the same level of credibility.
Then the same is true of Sulpicius. Primary source for events in his own time, and probably reliable for them. Secondary source for things he's only heard about, and no way to know whether he is reliable.
We know Gregory used Sulpicius because he tells us so, and he tells us some of what he got from Sulpicius. But he also says Sulpicius does not name their first king, then he tells us some stories he got from Sulpicius. No connected genealogy, just a few names of leaders. There's no indication from Gregory that Sulpicius had any more than that.
These are the names that later generations used to create a pedigree extending back 1500 years to the kings of Troy.
In the end, Gregory starts his connected history with Childeric, son of Merovech. He brushes off Merovech's ancestry with a short statement that "Certain authorities assert that king Merovech, whose son was Childeric, was of the family of Chlogio." Gregory had mentioned this Clogio a few sentences earlier as a king who had seized the city of Cambrai and land as far as the Somme from the Romans.
This is all relatively straightforward, but somehow you're turning it into an elaborate game where you're spinning elaborate conspiracy theories about the sources being mishandled.