For this you should consider Professional series devices (FMB/C/M640); such devices have external high gain GSM and GNSS antennas.
And please note that even there is not GSM signal, device saves all generated records into its internal memory.
Every record includes the "GPS element" data that is composed by: Longitude, Latitude, Altitude, Angle Satellites & Speed. But also the same record will include the "I/O Element" data (Event I/O ID)
Once device recovers the GSM signal, all those records will be packed and forwarded to server.
Easy, Advanced and Special trackers offer 128MB of internal memory.
Professional series: 2MB internal flash memory and external Micro SD card up to 32 GB
Hope this can help you