We know Pope John Paul II focused on preaching and teaching. But the bishops were also busy in the intervening years between the GCD and the promulgation of the GDC:
6. On the other hand the General Assemblies of the Synod of Bishops, both ordinary and extraordinary, have been particularly important for catechesis. In this respect mention must be made of the Synods of 1980 and 1987 which dealt with the mission of the family and the vocation of the laity. Following the work of these Synods, Pope John Paul II promulgated the respective Apostolic Exhortations Familiaris Consortio (22 November 1981) and Christifideles Laici (30 December 1987). The Extraordinary Synod of Bishops of 1985 was also of decisive importance for the catechesis of our times and for the future. On that occasion, following a review of the previous twenty years of the application of the Second Vatican Council the Synodal Fathers proposed to the Holy Father a universal catechism for the Catholic Church. The proposal was most favourably received and made his own by Pope John Paul II. After a long and complex process of elaboration the Catechism of the Catholic Church was presented to the bishops and the Particular Churches by the Apostolic Constitution Fidei Depositum of the 11 October 1992.
And these efforts indeed led to Cardinal Law’s proposal for a Catechism, the most high-profile product of these episcopal initiatives.