Does gatekeeping require conspiracy?

No.

Suppose an editor says:

“I rejected this paper because its evidence is inadequate.”

The editor may be acting completely sincerely.

Yet the decision has a gatekeeping effect:

The paper does not enter that journal.

That is why we need to distinguish:

Intention

Did the editor deliberately try to suppress this viewpoint?

from:

Effect

Did the editorial decision prevent this viewpoint from entering an important scholarly channel?

A gatekeeping effect can exist without intentional ideological suppression.

However, if you want to claim:

“The editor deliberately suppressed this interpretation because it challenged liberal nationalism,”

then you need evidence of that intention.