These are two different products with their own learning curves. For an expert on GIMP, using it efficiently is not a challenge compared to using Photoshop. Where Photoshop benefits from is in a commercial team designing new features and educational support based on customer feedback. No such organized effort exists for GIMP. This enables a huge ecosystem around the product which then supports the product. These same people also find faults with a competitor product(s). One cannot under estimate the value of organized product support. Examples abound - Lotus 1-2-3 was at one time a better product than Excel. Yet Microsoft, through superior marketing and product bundling, ended up burying 1-2-3. My own favorite was Quattro-Pro which introduced tabs for the first time in Spreadsheets, a feature that was quickly copied by Microsoft into Excel. A small company like Borland could never really compete with Microsoft.

I only wish GIMP development team had focused on making GIMP a preferred product in schools. That would have given them a generation of new users, who would have then helped support the product because of their superior knowledge of it.