Online collaborative writing is an increasingly common practice. Despite its positive effect on productivity and quality of work, it poses challenges to co-authors in remote settings because of limitations in conversational grounding and activity awareness. This paper presents Eye-Write, a novel system which allows two co-authors to see at will the location of their partner's gaze within a text editor. To investigate the effect of shared gaze on collaboration, we conducted a study on synchronous remote collaborative writing in academic settings with 20 dyads. Gaze sharing improved five aspects of perceived collaboration quality: mutual understanding, level of joint attention, flow of communication, level of negotiation, and awareness of the co-author's activity. Furthermore, dyads whose participants deactivated the gaze visualization showed a smaller degree of collaboration. Our findings offer insights for future text editors by outlining the benefits of at-will gaze sharing in collaborative writing.