England Hockey have experienced 'technical issues' with the GMS! This is a summary of their latest update:
Registration
Player Registration will be paused for at least 48 hours. We will communicate with clubs when this is ready to resume when system performance has improved

Officials and Club Admin roles will still be able to register on GMS and update key details. England Hockey is up to date with Club Admin approvals where people have been able to register.Officials should continue to register to England Hockey Officiating (EHO) via the GMS portal and declare their availability so appointments can be made by Area Officiating Committees

Because Player Registration is paused, clubs will not be expected to populate Match Sheets with player names until matches on the first weekend in October (apart from teams in the England Hockey Leagues, who we will communicate with separately)

Match sheets will initially be used simply to record the key match details such as the score. Guidance will be issued on this next week.

As a result:
TEAM captains must register as Players as soon as the GMS re-opens so we can allocate you as Team Admin (Coaches or Vice-Captains too if needed - teams are allowed 2 Team admins).
Anyone who may umpire a game needs to register as an 'official' in order to be in the system too.
https://www.englandhockey.co.uk/media/news/game-management-system-gms-update-9-september

We will try and keep you updated !