It is trivial to find them online, just search the company name and then "ads". I assume you don't go on TikTok or Instagram or if you do you aren't part of the target market to be served these ads.
The paper says that influencer marketing was new in Gymshark's case. It doesn't claim that this works repeatedly. Was it lightning in a bottle, i.e. Gymshark got lucky because it was early, or is it a consistently smart strategy?