I don't think he is surprised, although I see how one could be mislead in that scene to believe so.
I think Gandalf takes a moment to feel comfort about the fact that Sam is going with Frodo. Like an "Ah, yes." type of moment, if you know what I'm trying to get across.
Tis difficult to describe.
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"All rivers go to the sea, yet never does the sea become full."
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