Ever the materialist, Gould's assurance is entirely vested in the "grapple" (i.e, struggle) against things -- the mine and its attendant politics. When it comes to knowing how to "grapple" (i.e., hold fast to) another human being, the assurance deserts him. Note in the following paragraphs that he never says "I love you," although she does. From the several hints in the novel about his sterility, we may read his "distress, incertitude, and fear" to apply to the prospect of physical as well as emotional grappling.
There is also an allegorical level here, relating to the necessary marriage
of materialism and altruism that I elaborated on above.
This is the moment at which materialism (in the person of Charles Gould) resolves
to become the vehicle for a vast crusade of moral rehabilitation. In order to
function as that ideal -- in order to move masses of people rather than masses
of material -- it must seek the affiancing of altruism, which alone has the
necessary moral dimension. The "incertitude" of Gould here is the
incertitude of materialism wondering whether altruism will "love him enough"
to lend moral sanction to a materialistic crusade.