The rise in sea levels is extremely slow at human lifetime scale. We are looking at less than 1 meter (just over 3 feet) by year 2100 in the worst case scenario analyses by IPCC. This is not nearly enough to make Gibraltar power plant practical, as it is on the order of magnitude of existing difference between high and low tide.

Since parts of Northern Europe still experiences Post-glacial rebound and will rise faster than the expected rise of the ocean level I find this project unnecessary. (I am still a sucker for megaprojects though)

On the other hand, parts of Western Europe also still experience Post-glacial rebound and will drop, increasing the effects of rising sea levels.

https://en.wikipedia.org/wiki/Post-glacial_rebound#Vertical_...:

“the post-glacial rebound of northern Great Britain (up to 10 cm per century) is causing a corresponding downward movement of the southern half of the island (up to 5 cm per century)”

IIRC, the Low Countries also are sinking because of post-glacial rebound.