Use:
:%y+
to yank all lines.
Explanation:
% to refer the next command to work on all the lines
y to yank those lines
+ to copy to the system clipboard
NB: In Windows, + and * are equivalent see this answer.
You should yank the text to the * or + registers:
gg”*yG
Explanation:
gg to get the cursor to the first character of the file
“*y to start a yank command to the register * from the first line, until…
G to go the end of the file