In GDB, how can I jump directly up to the topmost call stack frame?

You can specify 'bt' with a negative number to start from topmost frame:

bt -20

You can then use 'frame' (or 'f') to directly go to the frame you wish.

In my tests, using 'up' with a very large number resulted in the topmost frame being displayed, e.g.

(gdb) up 99999
#58 0x0000000000442fb4 in main ()