**4. Conclusions**

According to the influential factors and the evaluation methods, recent studies of GSP have been investigated over the last twenty years in this review article. It is concluded that:


### **5. Future Studies**

Based on the current literatures, some recommendations for future studies are provided, as follows:


**Author Contributions:** Conceptualization, X.G. and P.H.; methodology, X.G.; validation, X.G. and P.H.; formal analysis, X.G. and P.H.; investigation, X.G.; resources, X.G.; data curation, X.G.; writing— original draft preparation, X.G.; writing—review and editing, X.G.; visualization, X.G.; supervision, P.H.; project administration, P.H.; funding acquisition, P.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** This work was supported by the China Scholarship Council (Grant No.201906560026).

**Conflicts of Interest:** The authors declares no conflict of interest.
