Let us first clear the meanings before trying a translation. First off: you added "gefährlich" (dangerous), but that wasn't even necessary. Both words, "Halbwissen" and "Halbwahrheit" have strongly negative connotations and both are considered to be bad. The element of dangerousness is already implied and only emphasized by adding it explicitly.
- Halbwissen: means the problematic state between complete layman and expert. A layman would shy away from doing something, an expert would do it but would know all the problems maybe arising from doing so and take them into account. Somebody with a "Halbwissen" will know how to do it but lack the experts circumspectness, causing a catastrophe.
You may have heard about the Dunning-Kruger-effect where people with no competence at all know they know nothing and experts tend to underestimate their competency level but people in the area between tend to overestimate their competence. This is what the word is about.
- Halbwahrheit: means something not outright wrong but lacking certain parts of information (usually to spin something). "Half-truth" is a good translation. If I tell you that I own a big company, it will leave the impression that I am rich. If I would add that the company is bankrupt that would change this impression dramatically. So, telling only the first part would be telling a "Halbwahrheit" or a "halbe Wahrheit" - not a direct lie but not the whole truth either.
To answer your question: "gefährliche Halbwahrheit" (the genus is female as "Wahrheit" is female and geman compound nouns always have the genus of the last part) can be used and sounds perfectly idiomatic, although one could leave out "gefährlich" (dangerous), because this is already implied. Whoever tells "Halbwahrheiten" has malignant intentions anyway and should be considered dangerous by default.
"gefährliches Halbwissen" (das Wissen is neuter) is a common phrase in German and describes exactly what you said: doing things with superficial competence and the danger that can cause. One can use it without the "gefährlich", e.g.:
Auf dem Gebiet habe ich nur Halbwissen.
meaning "I am (somewhat knowledgeable, but) no expert".
A probable translation (but without the connotations that has in German) would be "smattering". English is not my native language but "half knowledge" is, according to my dictionary, also an idiomatic expression.