Usually your common areas are accessible due to the fact you pay for that right in your condo fees. I would contact your attorney you closed on your condo with in regards to this problem. He can easily review the condo docs to see what your rights are to accessing the garden. Your agent may also have the condo docs, and if he does you could review them yourself.
Hope that helps
Hopefully you received a full copy of condominum documents when you purchased your unit, inclusive of Master Deed, Trust, and By Laws. I would then suggest that you have them reviewed by a real estate attorney. In most condominiums common areas such as pools, exercise rooms, garden areas are accessed by unit owners with a master key. Your attorney can assist you in finding whether or not the management company can change the lock to common areas without notification to the owners.