A sacred gear is an object containing the power, abilities, and soul of some mythical creature defeated and imprisoned within by God of the Abrahamic faiths.
The gear is placed securely in your soul and while the creature can communicate with you, it is sentient and sapient, and shares its supernatural essence with you. It cannot in any way harm you without your consent.
You can pick any sacred gear shown in canon including the Longinus class gears. You only start out with their basic abilities and forms and have to work your way up to their true power, balance breaker states and beyond.
Any sacred gear you pick that is not Longinus class has the ability to evolve into that level of power.
Be aware that while they are securely contained within your soul, their essence still somewhat leaks out into you, leading to a person having a draconic sacred gear to becoming a bit more draconic themselves, and gaining a faint dragon like aura as an example.