Does the program have the gravitational constant built in, and how is it referred to if I use it?

It is 'G.N

You can figure it out opening the units' dialog and typing something in the quick search field (CTRL+W or "Insert" > "Unit..." or click on the blue funnel on the toolbar)

Wrote

Does the program have the gravitational constant built in, and how is it referred to if I use it?

As replied by Davide.
Otherwise it is not a resident constant as not being a constant
like pi, e ... just an agreed value 9.80665 m/s²
Unit menu => Acceleration => Insert ... evaluate.