You're kidding right?
GSM, Wifi and Bluetooth controlled relays are everywhere. You can pick them up in Maplin's if you want.
And, no, we're not just talking solder-together kits but proper pre-built and support modules where you just power the thing, and then put the "button" wires through the relay. Job done.
Hell, for £10 on Amazon, I bought a GSM-controlled device that monitors GPS (even when the car is off), texts me if the car moves, tracks the car in real-time, alerts on overspeed, etc. as well as allows me to listen in to a microphone in the car, text the car and cut off the fuel pump, etc. by relay (included in the box!). It was literally cheap Amazon junk that can do exactly what you're asking.
I also have an house alarm that does the same (text it to activate a relay as a side-effect of it's 8-zone PIR / reed switch, battery backed house alarm functionality).
There are also bluetooth sensors that can activate when a particular bluetooth device comes in range / leaves range - sounds ideal for that purpose, no messing about with apps, just driving up to the garage with your phone switched on and it opens up.
Hell, my garden gate is maglock with an RFID reader and my girlfriend and I both have RFID tags. She uses it from her bike (push bike, mind) so that she can cycle up, fob the reader without having to get off, then ride into the alleyway directly. It cost me £10 on Amazon too, and came with a bag of fobs. If she loses the fob (like if your phone app doesn't work!), she just types in a code instead.
There's no need for poking holes in firewalls or other junk if you buy the right stuff. Which is often the cheapest junk because it has no subscription or dependence on a massive server infrastructure to do a simple job.