Most gate systems output audio as part of a POTS line and if there is CCTV is a basic analog camera. Opening gate is typically by relays. The biggest issue may be digitalizing the analog line from the gate, but Crestron does have door phone interface that may work here. I've not used it and I'd recommend testing it in house before attempting the installation as my experience is that there are always issues.

For example, the video string in the NVS-200 setup screen will work on an iPad but not the TSW panels. Interfacing this 3rd party doorphone may not be as transparent as it might first appear.

You may need worry about surges unless you're using fiber to carry these analog signals. It's never as easy transcoding signals as it is made to seem. If you've not done RAVA before it can be a ton of fun as well. I'd mock it up in house before attempting it in the field.