You can use the {site} command to information from the site you are on:
In the case of Gmail you should be able to find the name of the author of the email you are replying to using the following CSS selector:
Hello {site: text; selector=.h7:last-child .gD},
...
Note the error message above about "No match" is because the example snippet naturally can't find the name when running in the community forums.
If you wanted just the first name, you could split the string on the spaces and take just the first item like so:
Hello {=split({site: text; selector=.h7:last-child .gD}, " ")[1]},
...