The voice chat in Gray Zone Warfare has two modes: proximity chat and squad chat.
When not in a squad, you use only the proximity chat. Players around you can hear you speak up to a certain distance as if you were speaking with someone in real life. This mode only works while connected to a server.
Then, there is the squad voice chat. This mode works automatically when you join a squad. Your squad mates will hear you regardless of your distance from them or even when you leave the server and stay on the main menu. This mode simulates radio communication, and all voices transmitted through the squad chat are modulated as if you were speaking through the radio. Other players close to you will hear your regular voice if you are within hearing distance.
You activate voice chat by pressing and holding the Z key. You have up to 15 seconds of speaking "budget," after which you need to wait a moment to recharge before speaking again.