As FreeSWITCH allows you to place XML configuration almost anywhere you will need to decide where you wish to place your gateway configuration.
It is suggested to place a file called thinktel.xml under conf/sip_profiles/external/

In a default configuration, the <gateway> head tag exists in the main sip configuration file just before the external directory include statement.

<gateway name=""> # replace as required
<param name="username" value="XXXYYYZZZZ"/> # your SIP authentication username
<param name="realm" value=""/> # replace as required
<param name="password" value="@@@@@@@@"/> # your SIP authentication password
<param name="expire-seconds" value="3600"/>
<param name="register" value="false"/> # registration is not required/supported
<param name="caller-id-in-from" value="true"/>
<param name="ping" value="30"/> # will mark the gateway down if it cannot be pinged