The Memory Gap depends on the order that was given mainly if the order strips away the free will of the target, and example of this is when we see Mao ordered to
Never speak again!
Mao is unable top speak yet his will remains otherwise his memory would be a mess because of the constant order, however with Lelouch commanding Villetta to hand over her Knightmare it stripped her of her free will so she did not remember after the geass dispelled. With the girl doing the crosses on the wall the order was
Make a cross-shaped mark on the wall every day at a specific time.
This would mean that the Geass lays in the subconscious for the specific time where it will activate and the girl would make her way to the wall, only after she made the cross and returned would she regain control but not remember what she did due to her free will being taken away.
With Euphemia's accidental order to "...kill the Japanese..." we see that she remembers the order when she fights against it as when she is dying the Geass activates again when she remembers Suzaku is Japanese however after she shakes it and remembering that she was thinking of killing Suzaku however she does not remember ruining the ceremony which see moment after when she asks Suzaku how it went, this hows that the free will have to be totally stripped from the target for the memory gap to occur (Euphies free will was stripped before she was shot but not stripped when she was thinking of killing Suzaku)
We have seen people ordered to forget, the first instance is with a unnamed male artificer who made the Zero Costume, the order was
To make the Zero costume, destroy the evidence and forget everything about it.
This would indicate that there is a chance that one can find evidence and help them remember as we see when Suzaku listen to what was recorded from the Lancelot's com radio after Lelouch ordered him to Live. We also see Lelouch ordering Shirley to forget about him after her emotional breakdown caused by Mao so it stands to reason that if Lelouch can make someone forget he can make them remember, an implementation of this is seen with Mao when Lelouch uses Geass on himself to to forget his plan with Suzaku to prevent Mao reading his minds and remembering it when he looks at himself in a reflection