Hi,
Thank you for your answer.
I've asked Hikashop already. According to them, there is a problem with GCK Store template.
When I switch to joomla default template "Cassiopeia - Default", the icons are displayed without any problem, so it's not a issue from Hikashop.
Is GCK Store able to use default Font Awesome library included in Joomla ?
I've noticed that Font Awesome does include Font Awesome 3.2.1, when I disable these library it doesn't seems to be able to use FontAwesome 5.15...