gistfile1.cpp

// Lower end of ASCII printable (space)
#define ASCII_PRINT_LOWER = 0x20

// replace.
char[] want_kind = "$GPXXX";
int want_idx = 0;

void loop()
{
    /* stuff to keep strobing happy */
    
    // if we have serial data
    if(Serial.available())
    {
        // check to see if it's what we want
        // if idx = 0, want_kind[idx] = $
        // if we get $, we go on, looking for G..
        if(Serial.read() == want_kind[want_idx])
            want_idx++; // the stanza we're going to get looks kinda like what we want.
        else
        {
            // the stanza we are going to get is NOT what we want.
            want_idx = 0;
            // eat anything until we get a newline, or anything that's less than 
            // the lower end of ASCII's printable region.
            while(Serial.Read() > ASCII_PRINT_LOWER   ) ;
        }
        
        if(want_idx == sizeof(want_kind)-1)
        {
            // we have the stanza we want.
            // parse it as we need
        }
    }
}