All field keywords beginning with html_form_ are passed directly into the form hash. Thus, they are really defined in form.tt which ships with Gantry.
html_form_
Use html_form_raw_html if you need to put HTML directly into the entry form's table. Your html will be immediately before the <tr> for the current field. For it to appear in the table, you need to wrap it in a <tr>. Clever tricks are possible. Remember to quote carefully. Some experimentation might be required.
html_form_raw_html
To see an example, build:
bigtop -c example.bigtop all
Change to the newly created Kids directory and look in the form method in lib/Kids/GEN/Child.pm, but run app.server and point your browser to http://localhost:8080/child/add to see the result.
To install Bigtop, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Bigtop
CPAN shell
perl -MCPAN -e shell install Bigtop
For more information on module installation, please visit the detailed CPAN module installation guide.