If it be in the dusk when, like an eyelid's soundless blink, The dewfall-hawk comes crossing the shades to alight Upon the wind-warped upland thorn,
Lines 5-7 elaborate on the scene: it's "dusk," it's a lot gloomier than the springtime scene of the first stanza, and a "dewfall-hawk" (i.e., a hawk that comes out at "dewfall," or dusk) flies as quietly as you blink (completely silently, unless you hang castanets on your eyelashes) and lands on a shrub.
No, "dewfall-hawk" isn't a real species of hawk, so don't bother looking it up in your bird guides.
In line 5, it's not immediately clear what the "it" is that the speaker is referring to in the first line of this stanza, but he seems to be referring to the moment of his death, and imagining that "it" will take place at "dusk".
[…] a gazer may think, "To him this must have been a familiar sight."
Again, the speaker repeats the question of whether people will remember him, but this time, it's not phrased as a question, but as a statement.
He's still not certain that he'll be remembered, though – the whole stanza is a hypothetical statement ("If…"), and he only says that the "gazer may think." Not nearly as confident as the "gazer will think".
Also, he's switched from imagining what his "neighbours" will think of him to what the more anonymous "gazer" might think.