it is obviously included in the rent. when the tenant rented the property the price they agreed to included the security of the gate. You need to give them legal notice it will no longer be included. in most states that is 1 full rental month, so if you notify them in July, you can take it down in September.
if its an immediate danger, you can remove it right away and compensate them for the lose, maybe 1-5% of the rent.
i don't see why you have an access problem, you should have a key and be able to open it.
you could issue a cure or quit notice for blocking the driveway and evict.
you could send written notice to change in terms of lease, instituting a fine when access is denied. charge them full cost of the visit from the gardener or handyman.
if you want them out, and tenant laws don't offer any protection against lease termination for no cause....just send a notice to terminate tenancy. In NJ for example, you could not do that unless you have established they are a nuisance by blocking the drive way.....you would need to give them notices about that 1st and move to terminate based on that.,
you can tell the tenants to pick a time that is good for them to have the gardener come.....when the cars wont otherwise be in the driveway (during normal business hours), that way they have no excuse as they picked the time.