Gmail is an email service provided by Google. Since Google is a Body corporate, it’s essentially a Proper Noun. We should consider writing Gmail instead of writing gmail.
<!-- English rule, 2016-10-18 -->
<rule id="PROPER_NOUN_CAPITALISE_THE_FIRST_LETTER_WRITE_GMAIL" name="Proper noun. Capitalise the first letter. Write Gmail.">
<pattern case_sensitive='yes'>
<token>gmail</token>
</pattern>
<message>Proper noun. Capitalise the first letter. Write <suggestion>Gmail</suggestion>.</message>
<short>Possible spelling mistake: Write Gmail.</short>
<example correction='Gmail'>You are not signed in to <marker>gmail</marker></example>
</rule>
Please add it to LT.