Broadly, there are low assist motors like the Mahle ebikemotion system and higher assistance models like those from Bosch.
The former will help you out on less aggressive gravel rides and will keep the bike's weight down, and so provide more agile handling. You'll have to do more of the work to keep yourself moving though.
Then there are the high assistance models, which provide higher torque. They'll get you up and over tricky terrain more easily and with less effort on your part. But they'll be significantly heavier and need larger batteries, which can affect the bike's handling.
It's worth considering the type of gravel riding that most interests you and the terrain you like to ride, as well as how much support you want, and shortlisting electric gravel bikes that suit that need.
If you want more information on gravel bikes in general, we have a guide to what is a gravel bike, the differences between a road bike vs a gravel bike and gravel bike pros and cons to help out.