Don't know about Gigster but the first step of Toptal's vetting process is about solving algorithmic puzzles. That's probably inevitable because they need something that scales. However it's so different from what developers do at work that its relationship to quality is very weak. Actually, it could be negatively related because I would have excelled at those puzzles at university, exactly when I didn't have the expertise to write good software. And the one or two of times per year when I have to solve a puzzle I take my time and solve it. Maybe slower but the rest of my work is so much better that it more than makes up for that.

Want a better vetting quality? Look at previous work and read code. It doesn't scale unless you put more people at that but maybe somebody can think about an AI for that.