Provided they work a mixture of collets from different makers wouldn't worry me; it might even be a good thing! I suspect Gloster bulk buy collets by size for sale individually and box them up when customers want a set. What's in the set could be from several makers, all equally good.
Coming from a single maker (unless you know who it is) doesn't guarantee "quality" Perfectly possible to buy a too-cheap set of collets all made in the same factory.
DIN6499b specifies the dimensions of ER32 collets. It doesn't address quality.
In the good old days, decent tools were expensive and it paid to search out reliable brands because poor consumer protection made mistakes painful. Not so today. Now, we can buy mid-range tooling from firms like Gloster, and send them back if they're not 'fit for purpose'. I suggest Richard tests his for run-out and fit. If they're OK, they're OK.
Dave