I personally haven't played with using r to send emails in years, but I am pretty certain you can just use html_body
for html markups.
Here's a vignette https://cran.r-project.org/web/packages/gmailr/vignettes/sending_messages.html
mime() %>%
to("james.f.hester@gmail.com") %>% #> i can't believe he put his personal email here
from("me@somewhere.com") %>%
html_body("<b>Gmailr</b> is a <i>very</i> handy package! Check out <a href='example.com'>this link</a>.") -> html_msg
I really like the compose-and-send-your-emails customization of Jenny's tutorial. Great tip!