This cannot be changed using Customizer options or some code snippet that we could provide. To add/change this feature to your website, the best practice is to create your own child theme -starting with gwangi-child- and override the BuddyPress Register page template:
For more about creating child themes, please read the following post:
If you have never done this before, we recommend you to require the services from a nearby web developer or from Envato Studio freelancers (https://studio.envato.com/explore/websites-programming). This web platform gives you access to developers who have already experience with theme customizations, and for very competitive prices.
Maybe these topics will help you to solve your issue as they seem to answer the same concern:
Please note that this issue is related with the customization of a third party plugin, not with the use of your theme. As theme authors, the installation, setting or debugging of third party plugins isn’t part of the support we will provide.
To go further, we invite you to ask for more help from the plugin authors and post your request on their support forums:
Finally, here is some specifications:
The Kirki plugin is an amazing customizer toolkit that we use within Grimlock to create and manage all the fields in the WP customizer. The Kirki plugin will do nothing on its own and Grimlock will not work without Kirki so both of them are required for your theme.
In conclusion, Kirki and Grimlock give you the possibility to customize your theme. For more about this, here is an article from our documentation:
We remain available for all support requests related with the use of your theme.