Welcome! I’d check out this thread for the GE Link bulbs. There is an improved device type, and post 51 or so has step by step instructions for adding the new device type code to your bulbs.
In general, the device type code in that thread will use transitionTime when changing the dim level, so a command for setLevel(75) would slowly move the dim level to 75% from wherever it started (1.5s transition time from 0-100%). This code does not use transitionTime for On/Off commands, so if you just tap the icon to turn on, it will move directly to the last dim level. That is controlled by a transitionTime attribute, but we are having trouble getting it to stick. Not sure what use cases you have in mind, but if you always use setLevel() even for on/off (ie setLevel(99)/setLevel(0)), you will get the slow movement you’re looking for. (Of course physical state changes via a wall switch will turn the bulb on/off instantly as well.)