Research gap means that your thesis can supplement or even overturn the shortcomings of previous research in this field. That is, what new value your paper has created.

Research restriction means that although your research can support your inference, you understand that your research or experiment is limited by reality or some restrictions, so you can't cover all or be proved, so explain these restrictions in the research restriction. For example, the number of samples is not enough or the research time is not long enough. Another good example is the Higgs boson inferred by physicists, which exists in theoretical physics, but the accelerator enough to find particles has not been invented, and the God particle was only proved 50 years after the publication of the paper.