The GPS system list just specifies the contact to which some positional information is send to (which usually gets forwarded to the APRS system) and at which period this information is send.
GPS Name Dest Period Revert 1 "BM APRS" 20 300 -
The first column specifies the ID of the GPS system. This can be any number >0. The second column (Name) specifies the name of the GPS system. The third column specifies the destination contact ID (see Contacts above), the position information is send to. The fourth column (Period) specifies the update period in seconds. The fifth column (Revert) specifies the revert channel. In amateur radio, this can be left blank ("-").