Adverbs are lazy.

Would suggest "and the most innovative workout shirt designs..."

Cliched, something like "Performance boosting" might be better.

This uses personal pronouns throughout and then goes into second person-perspective here.

It would be more consistent to say "It's not just in the designs, it's in you."

Unnecessary.

Unnecessary. Just start the sentence with "Every..."

Run-on sentence. Would edit as:

"The Gymshark community is devoted to unlocking potential through conditioning. We believe the things we do today prepare us for tomorrow.

Compound adjective here should be hyphenated (rest-day wardrobe)

Too punny. The brand is cooler than this.