Hello,
DSpace SMTP settings are described in the documentation here:
https://wiki.duraspace.org/display/DSDOC6x/Configuration+Reference#ConfigurationReference-DSpaceEmailSettings
You can update those either in the dspace.cfg directly, or (as recommended at the top of that Configuration Reference) you can copy those settings into a local.cfg file (which will override any defaults in your dspace.cfg).
If you are specifically looking to configure DSpace to send email via *Gmail*, that can be accomplished via the following settings in your local.cfg:
# Mail Settings to support Gmail
mail.server.username = [your-gmail-username]@
gmail.commail.server.password = [your-gmail-password]
mail.server.port = 465
mail.extraproperties = mail.smtp.socketFactory.port=465, \
mail.smtp.socketFactory.class=javax.net.ssl.SSLSocketFactory, \
mail.smtp.socketFactory.fallback=false
There are also additional mail settings that are documented in the Configuration Reference (linked above) and can be found in the dspace.cfg by looking for settings starting with "mail."
Good luck,
Tim