I've been trying to do the same thing myself (on 5.24.1), and you're right. There's absolutely nothing out on google. So I submitted a bug via the help menu. Well, the project hasn't used that specific bug tracking system for several years. So after some heavy investigative journalism, here's the bug-tracking page for the project:
http://bugzilla.gnome.org/browse.cgi?product=gcalctool
The 2 related bugs are in the Components => docs area. They're still not fully addressed yet. However, the key is adding an "=" after the statement. "sin(45)=" works properly, as does "sin(R0)=". R0 of course being a stored value.
It's a heck of a thread bump; but it was important to me, and it answers the question properly.