In some modern media it has come to be treated as innately sarcastic and mistrustful, but that is not its actual meaning or original use.
There’s a dark comedy from 1909 called “Charmed, I’m sure” where the phrase is meant as sarcastic; that use has been around for more than a century.
Also, it’s not elegant to specify that what you’re saying is true. There’s an implicit assumption that stuff you’re saying is true. Another assumption about language is that what you’re saying is relevant. Specifying “this statement is true” is irrelevant if the statement actually is true; it’s only relevant if you’re being sarcastic.
Edit: maybe I shouldn’t be so categorical. In truth I’m not positive about this interpretation. Another explanation could be Gale saying “I’m sure you’re charmed by me”
