GDPR in the workplace is how data protection law applies to workplace systems: booking platforms, visitor logs, access records and attendance data are all personal data processing, and UK and EU GDPR set the rules for it.
The practical duties: a lawful basis for each use, proportionality (collect what the purpose needs, no more), transparency with staff and visitors, retention limits, and the ability to answer subject access requests. Digital visitor books beat paper here: a paper log on the desk shows every visitor the previous visitors, which is itself a breach waiting to be cited.