convert pow gradient to new style

[t-vi]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• n/a Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Tries to fix #2282 .

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[t-vi]

Merging this to get some CI fied.