Hello L.,
For that you can use style, as:
@startgantt
<style>
caption {
HorizontalAlignment left
FontName Serif
FontSize 19
FontColor green
FontStyle italic
BackGroundColor yellow
}
</style>
[Prototype design] lasts 15 days
[Test prototype] lasts 10 days
then [T2 (5 days)] lasts 5 days
caption
that is
a caption
end caption
@endgantt

If that can help,
Regards,
Th.