Should arguments based on the proponent's gender be used in a grant proposal? Recently I have reviewed a project from a female proponent. Among the scientific content there was a section about the under-representation of women in a specific field - saying that, on top of all the scientific justifications for the project, it should also be approved to support women in that field.

I know that in some countries, e.g. Germany, gender is often considered a tiebreaker as affirmative action in order to support women in science. However, in this case there is no such policy in the call guidelines. 

Is this kind of argument appropriate in a grant proposal? Should it be a criterion to be taken into consideration?
This does not feel like something you as an external grant reviewer should really care about. The gender of the applicant may or may not be a tie breaker (or even a major factor) for the jury, but they will know which it is. If gender is a factor in the evaluation, they will also be acutely aware that the applicant is female, because either the system or an administrative person will have flagged the application as coming from a female applicant. No need to tell the jury that this is indeed a female applicant.

In consequence, I think you should simply ignore this part of the application. Keep in mind that as a reviewer, you are not deciding on the application - you are merely laying down the facts (e.g., related to the scientific quality of the proposal and applicant) so that the jury can decide based on their own strategy, criteria and availability of funding.

Side remark: note that even should you disagree with the notion that the gender of the applicant should play a role, you should resist "mentally subtracting some points" because the applicant even brought it up. It's not your place to decide if gender should or should not be part of the evaluation. If you would decide to judge the proposal more harshly to counteract a perceived undue advantage, you yourself quickly become part of the problem. Nothing good can come out of that.