The GDL is a one year postgraduate course offered by various providers, not just the two mentioned above.
The core subjects are -
Public Law (Constitutional and Administrative Law)
Criminal Law
Contract Law
Tort Law
Equity and Trusts
Land Law
EU Law
After taking the PGDL, you would take the SQE if you wish to become a solicitor, and would need to obtain a training contract with a law firm. If you wish to become a barrister, you would need to take the Bar Practice Course and obtain a pupillage in a set of chambers.
You don't have to do a PGDL before the SQE, but it is thought to be a good idea to do so.