@Pengzeus @MuteLunatic is right, it's just to prevent giving away your position when you're camping an area. At long distances that shield is especially bright, so in some instances it can be better to hide it, especially to pull off ambushes.
Shield health has its own personal health bar of 75. Once the 75 health is depleted, the shield turns off and you're left with your own standard health. After you stop ADSing, the shield will automatically start recharging. ADSing again while it's recharging will pull the shield up with whatever health it's had enough time to recharge. For instance, if you pull it up at the halfway point, your shield will be able to block about 37 damage. Please note that while you're ADS, your shield does not recharge. I'm not certain the rate of recharge his shield has, but supposedly using shield cells or batteries helps recharge it. https://apexlegends.gamepedia.com/Gibraltar#Info
Here's a tip while facing Gibralter, don't get suckered into the mindset that just because he's fat, he's an easily killed target. At a distance when he can't fire back at you, he is absolutely an easily killed target. Up close when he can take full advantage of his innate tankiness and gunshield, he is a massive threat. Adopt a quick peeking strategy where you peek quickly from cover to do damage then retreat once his gunshield comes up, throw grenades to keep him moving so he can't take advantage of his gunshield, or the best option is to comm with teammates and focus fire him down. He may be able to survive one full purple mag of an R99, but he can't survive two.
