If vim is compiled with the clientserver option, you can do it. Start your vim instance with the following flag:
$ gvim --servername GVIM # GVIM is the server name. It can be anything.
To open more tabs in this instance, you can run the command:
$ gvim --servername GVIM --remote-tab file1 file2 file3 ...
The clientserver feature in vim is very handy. It’s not limited to opening files; it can be used to send any command to vim using the command-line. For example, to close a vim instance remotely, you can use:
$ gvim --servername GVIM --remote-send '<Esc>:wqa<CR>'