You don't need J var. Just increment GLCD_Addr.
Code:
GLCD_StringOut:
Readcode GLCD_Addr, g_chr ' Get a character
if  g_chr = 0 then RETURN ' Look for Null char, Stop if found
GLCD_Addr=GLCD_Addr+1
gosub gcga 
goto   GLCD_StringOut
And If you need to clear variable, you always can use CLRF, it's just shorter then loading 0 to w then from W to variable.
And if you want to avoid CHK?RP, declare variable to BANKA.