Has anybody needed to replace the internal battery for the GPS? What
type of battery is it?

The GPS battery presumably runs the real time clock in the GPS module.
If this battery fails, when you start up the device the GPS screen will
not show the correct time of day until signals from the GPS satellites
are received. Also the GPS screen will not show the previous lat/lon
while it searches for the current location. And it will probably take
longer than normal to acquire a GPS location fix.

This is distinct from the internal coin battery for the IGC security,
which presumably also powers the memory that stores the user settings.

It seems that on my unit BOTH batteries need replacement. Does that
make sense (about 11 years since factory)?