I can't (be bothered to) make out what your intermediate output is supposed to be doing, so your BASIC code boils down to:

NEMA$ = "$GPRMC,000004,V,3714.6284,N,12146.9181,W,0.000,0.0,290697,15.4,E*4C"
PRINT " " + NEMA$
chksum = ASC(MID$(NEMA$, 2, 1))

FOR x = 3 TO LEN(NEMA$) - 3
chksum = ASC(MID$(NEMA$, x, 1)) XOR chksum
NEXT x

PRINT " NEMA Checksum is " + HEX$(chksum)

And the 'C' equivalent is, I think:

void main( void )
{
   unsigned char checksum;
   int x;

   char nema_string[] = "$GPRMC,000004,V,3714.6284,N,12146.9181,W,0.000,0.0,290697,15.4,E*4C";
   printf( " %s\n", nema_string );
   checksum = nema_string[0];

   for( x = 3; x < strlen(nema_string)-3; ++x )
   {
      checksum ^= nema_string[x];
   }

   printf( " NEMA Checksum is %02X\n", checksum );

}