Trace is basically the precedent we can follow to describe how N-Gas works against abilities like Intimidate. If Trace shows up against an un-traceable ability (like Schooling or RKS System), it doesn't announce, and instead waits until the first moment there's an ability out that it can trace before firing and taking that. Similarly, N-Gas vs. Intimidate on the lead matchup will have N-Gas fire first, suppressing the other ability so it can't fire. When Weezing switches out, the abilities stop being suppressed, and Intimidate then fires at that point if it hasn't already (in fact, it fires early enough that the departing Weezing itself is the Pokemon that gets the attack drop, which in singles causes the delayed Intimidate to be pretty much useless when it does get around to happening). This is different from the way Intimidate has worked before against a substitute, or against abilities that block it like Clear Body: in those cases it fires normally on entry, to no effect, and then never again for as long as it stays out.
If Trace is suppressed by N-Gas, it should naturally see all abilities as un-traceable, and only fire after the ability stops being suppressed. Whether or not it's possible to trace Neutralizing Gas itself on an attempting switch from Weezing remains to be seen, but I doubt it is.