Ensure you are using a compatible version of Gboard (beta versions are often preferred by the community for the latest features). Installation Guide Rboard Themes

To use (or similar .zip theme packs), you generally need a rooted Android device and the Rboard Theme Manager app. This manager acts as a bridge to inject custom themes into Gboard that aren't natively available. Prerequisites

You should have the manager app installed. It can often be found on platforms like XDA Forums or official Telegram channels.