thanks for your post... then.... IT WORKS! :-)
my GPS device uses Serial Port Profile (SPP) based on RFCOMM: I thought that perhaps was a problem of RFCOMM configuration in Ubuntu...
$ hcitool scan
00:0A:3A:xx:xx:xx BT GPS
$ sudo sdptool browse 00:0A:3A:xx:xx:xx
Browsing 00:0A:3A:xx:xx:xx ...
but nothing more... sdptool should give me the service, the channel, ...!!!
so I edited /etc/bluetooth/rfcomm.conf:
# Automatically bind the device at startup
comment "BT GPS";
$ sudo rfcomm connect 1 00:0A:3A:xx:xx:xx
Connected /dev/rfcommm1 to 00:0A:3A:xx:xx:xx on channel 1
shows NMEA sentences!!! :-)