Is it reasonable to ask candidates to create a profile on Google Scholar?

As much as I like Google Scholar, requiring candidates to create a Google Scholar profile specifically seems inappropriate. You are effectively saying you won't hire people that don't use Google.

What you could do is make it an optional part of the application or you could ask candidates to submit something more vague like a "citation report" and suggest that a printout of their Google Scholar profile is sufficient for this.

Have you considered ORCID (Open Researcher and Contributor IDentifier)?

I have the same concerns about intellectual property protection issues around Google unfortunately. GoogleScholar is also quite discipline specific (as others have said here) and is banned in some countries (China, etc). So to endorse a product that exposes a scholar to legal ramifications in their country plus the risk of commercialization of their data is highly problematic.

ORCID on the other hand is "an international, interdisciplinary, open, non-proprietary, and not-for-profit organization". ORCID aims to include every discipline and many publishers and their journals are now mandating ORCID sign-in for their journal logins. Most people do not know about ORCID so you can offer them information. Also, make sure they know to make their ORCID profile public.

Unfortunately, if your applicants refuse to use ORCID, your choice would be limited. I am not sure whether you have a friendly and supportive librarian can confirm their publication record and piece together their impact factor before progressing them through the selection process?

I do not think it is reasonable to ask candidates to create a profile on any third-party platform. Particularly on a google service, taking in account that some proportion of web users have concerns about this company (as well as other large corporate data processing companies), and do not want to get on their radar if possible.

Typically, it is sufficient to make it clear to the candidates what are the selection criteria for the post and let them find their preferred way of demonstrating that they meet those criteria. For example, if your criteria is number of citations, you can suggest Google Scholar as an acceptable evidence, along with WoS and others, ultimately allowing your candidates to choose the service they prefer. If you want to check impact, you need to explain what you mean by this (the definitions vary widely across different fields and countries). Note that impact typically is not measured by the academic citations, but rather by adoption of research in non-academic environment, such as industry, government policies, patents, etc.

In the UK, the impact is a key performance indicator in the Research Excellence Framework. It takes Universities a few months to prepare and evidence strong impact cases. I am sometimes puzzled when I see an entry-level faculty post requiring candidates to provide a fully justified impact statement. Maybe it is possible in some disciplines, but in my area (numerical mathematics) I find it difficult to trace, demonstrate and fully evidence the non-academic impact.