Both Popper and Grünbaum took issue with Freud's theory, but they critiqued it for different reasons, in which the main difference seems (to me) to come down to this:
Popper interprets Freud as saying that something (say homosexuality) being repressed by the subject means it's a completely hidden state, so we can never know the truth value of statements like "subject X is repressing his homosexuality". So Popper argued that psychoanalysis is untestable because testing inferences like "if P then Q" (e.g. repressed homosexuality can lead to psychosis) is impossible for we can never know the truth value of the antecedent (P).
On the other hand, Grünbaum considered that statements like "repressed homosexuality can lead to psychosis" propose testable etiologies. And it's not hard to conceive that (say) using a penile plethysmograph one can test for homosexual preference. Once we agree we can test for the latter (even if not with perfect accuracy), we can also test for (statistical) associations etc. Kihlstrom pretty much takes this view in listing most Freudian common "knowledge"/lore as false rather than unfalsifiable.
Of course, Freud's theory is screwed in either case (for this example, and probably for most other). Whether it's screwed as non-falsifiable from the get go, or as falsifiable but almost certainly false ... is not incredibly interesting to most people.
But for the sake of curiosity, did Freud make explicit what he considered to be knowability of "repressed"? I.e., did he assert about any repressed condition (as he conceived them) that we cannot really know whether it's really there? Is something called repressed by Freud if and only if it is a true hidden state? So, as an example "truly repressed" homosexulity could never show up on any plethysmograph, by this (Poppler-ian) definition of repressed. But did Freud explicitly define "repressed" in this absolute way?