The UK Government has ratified the Convention on the Elimination on all Forms of Discrimination Against Women (CEDAW). This obliges the UK Government to uphold the rights of women under CEDAW, to protect women against male violence and to support women who have experienced male violence and to eliminate harmful stereotypes (as well as a number of other things).
The CEDAW Committee takes evidence on how CEDAW is implemented in signatory states and raises concerns where women's rights are violated and then makes various recommendations in periodic country reports. Most states do take heed of these recommendations because they don't want to be accused of breaching the rights of women, but it is more or less voluntary, so states can and do ignore recommendations at times.
However, the UK Government typically seeks to heed the recommendations and the Scottish Government has been very vocal about its commitment to CEDAW.
The country reports on the UK, as produced by the Committee, stress that it falls to the UK Government to ensure that women's rights under CEDAW are upheld across the whole territory of the United Kingdom. Scotland is not a state actor, the UK is.
What this means is that it falls to the UK Government to step in if the GRR Bill fails to meet its obligations as a signatory to CEDAW.
Before anything else of course this is already the case, because of the refusal by both the Scottish Government and the Committee responsible for the GRR Bill to consider the reform's impact on women, something which is an important obligation states have under CEDAW.
TL;DR: this letter gives the UK Government a very good reason to stop the GRR Bill from being implemented.