As a gig worker I was an IWGB member for three years. I don't think it's yet generated enough momentum to be effective, either in terms of membership numbers or activism. They have had some successes, though they're also keen to sign members up for legal class actions on poor conditional fee agreements, rather than raising the money to pay the lawyers. In terms of courier benefits, if they win all couriers gain, those who were part of the action just gain less! I left when I was unhappy with the campaigns they chose to prioritise, joined the GMB when they set up a gig workers division.
If the cycle instructors are going to join forces and campaign together, then the IWGB may be a good flag to march under, but other options might be worth exploring. Like all activism, it just takes a few to make a lot of noise, but it takes a good proportion of those employed to speak as one before the employers listen. I'm not sure cycle trainers will ever be unionised enough to get there.