WD's G-Tech brand which they got through the HGST acquisition in 2012 is a very well-known Premium brand among Media Professionals and Mac users for the performance, features, and build quality.
Very well known, but not as well known as SanDisk. G-Tech still is a niche brand and SanDisk is mainstream. Being a consumer brand, the brand visibility and the number of consumers and channel partners selling SanDisk are all higher than G-Tech. And this vast SanDisk channel is what WD is probably hoping to tap into by changing the brand from G-Tech to SanDisk Professional.
Now how successful this rebranding would depend on the popularity of the thunderbolt3 devices. The M&E customers who are the primary users of these devices know G-Tech very well so there the rebranding wouldn't matter much. they will buy G-Tech or SanDisk irrespective of the branding as long as they get the features they want.
The incremental business could come from consumers who need high-speed high-performance thunderbolt or USB storage. Consumers who already trust SanDisk and will buy SanDisk more readily than the G-Tech brand. With the increasing popularity of Thunderbolt-3 and with more and more devices shipping with this port, the number of consumers looking for TB3 devices should go up as well.
Product-wise - nothing has changed for G-Tech products. It's the same great G-Tech that media customers trust for performance and quality - only rebranded as SanDisk Professional. However, the total SanDisk professional universe is larger now as it includes products from G-Tech and SanDisk Professional too.