A zigbee device can only belong to one Coordinator at a time. The smartthings hub is a zigbee coordinator, and so is the lightify Gateway. So a lightify bulb may join to one or the other, but not both.
Many people who buy lightify bulbs to use with SmartThings never buy the lightify Gateway. A few people do buy the gateway but only use it when they want to do firmware updates on the bulbs. And this process requires removing the bulb from SmartThings network, joining it to the gateWay, doing the firmware update, then removing it from the gateWay, and joining it back to SmartThings. Which is a lot of work.
Smartthings does hope to eventually add the ability to do over the air firmware updates, but it doesn’t have it now.
Most of the time firmware updates add support for some new feature, but if the bulb was previously working well for you, you may not care.
So you definitely do not need the gateway to use lightify bulbs with SmartThings. And if you do get the gateway, it will spend most of the time sitting on a shelf not connected to anything, and you will only use it if you want to update the firmware of your bulbs.