Assuming your Company is subject to GDPR, the level of effort depends a lot on the maturity of Organizational and Privacy controls. The Organizational will be mostly met by implementation of the related latest SOC 2 updates, which is good. However, if you are not currently including Privacy in your SOC 2, and do not have a strong privacy program in place, there will be a moderate level of short term effort required.
It should take approximately 2 to 3 months in collaboration with your Auditor to perform a readiness assessment of the specific GDPR requirements and implement said updates for most Company’s.
The upside of being required to implement GDPR is that there would be minimal effort to include Privacy and references to any unique GDPR requirements within future SOC 2 reports.