Made in the same factory. Aimed at the same demographic: ie the general purpose motorcyclist. How different can they possibly be?
The answer is way more than you might expect. Which puts to bed the widely held view that Metzelers are simply Pirellis rebadged to satisfy marketing demands. It would be useful if tyre manufacturers had a single, comprehensive data sheet in pdf format, inc. construction details for their individual products, but some information can be gleaned by simply going to the product webpage – certainly enough to know that we’re not comparing like with like.
I tested the tyres on the same bike and on the same roads, which is the only way to make a meaningful comparison. One of the few things they had in common was exceptional wet weather grip, which – particularly in the case of the Metzelers – did not appear to compromise higher speed, dry weather handling. Although the GT2s felt a tad keener to turn in (despite sharing exactly the same aspect ratio with the 01SEs), under hard acceleration at higher revs above 80mph (or thereabouts), the latter felt more stable – especially on questionable surfaces. And once leant over, particularly in fast bends, the Metzelers’ grip was almost Racetec like in its adhesion. I can vouch for the 01SE’s corner speed performance with some confidence: I spent a day going round Thruxton on the hottest day of 2021 with them. They were faultless: this might have had something to do with slick, full silica compound that comprises their shoulder, and characterisedstheir difference from their 01 predecessors. I didn’t have the opportunity to do a track day with the GT2s, and it therefore remains entirely possible that they would have matched the SEs. It’s just that on roads where both brands were tested I preferred the consistency of the Metzelers.
Both sets of tyres were tested two up with no problems. If anything, the pillion seemed to seemed to help iron out the slightly twitchy front end feeling I had from the GT2s under circs previously described. Which sounds counter intuitive, and I haven’t figured out an explanation – if anyone’s got any ideas, do leave a comment. The RS900 Cafe in use had its suspension set up by Racelab, which I adjusted slightly to accommodate a pillion. Suffice to say that both sets of rubber felt uncompromised by the extra load. 
CONCLUSION
The way we experience tyres is like the way experience most things: pretty subjectively. Contemporary motorcycle tyres are a miracle: those tiny, one wheel driven contact patches must and do handle an extraordinary plethora of variable parameters in order to help keep us all alive, and this in an age where the power figures can and routinely do reach >150 bhp. And then they have to cope with the braking forces required to bring the whole shooting match to a halt without incident. Many would say that pretty much all sports touring tyres currently on the market are so good that any of them would do a decent job – but that’s without the subjectivity factored in. We know they’re all pretty decent, but we prefer the feel of one set to another. And that has to be born in mind when considering the following.
The Metzelers felt more predictable, less disturbed by the vagaries of UK road surfaces. They also felt very stable on faster, big lean corners. They were excellent in the wet, but that was where the GTs also excelled, to the extent that if I was only riding in the wet they might get the call. Since that isn’t the case, and taking everything else into (subjective) consideration, the Metzeler 01SEs would be my choice.