You need a global array to hold the GPS data in, not a local array.
You need a global index into that array.
Each time you read a character from the GPS, there are three possibilities. The character could be the start of a packet - a $. If it is, set index to 0 and store the character.
The character could be the end of a packet - a carriage return, most likely, but the data after the * is just a checksum, so you could ignore that data and treat the * as the end of the packet. Whichever option you choose, the arrival of the end of the packet means that it is time to broadcast some data, after you store the character.
The third option is that the character is neither a start marker or an end marker, so it needs to be stored in the array. Store the character in the index position, increment index, and store a NULL in the index position.