I don't really care What you say. Just giving my opinion on something that doesn't make any sense either in reality or fictional.
This is not opinion you have, this is a hypocritical facade of yours with the only purpose to bash FT with non-sensical and irrational shiberish where you use the argument of 'it's just an opinion' as a feeble, pitiful excuse to post it.
Gajeel controls iron, he is iron and he literately eats iron for breakfast and has an iron breath. Steel is largely based on iron combined with a small amount of carbon, which normally is combined together with high temperatures so that they bind together creating the steel alloy. However Gajeel himself is iron, in fact living iron. So him absorbing carbon and making it bind together with his iron body to make steel is as sensible explanation as it possibly could be for this type of manga whether it be Naruto, One Piece, Hunter x Hunter, Bleach etc. and it counts double for Fairy Tail where 'non-sensical power-ups' are a common complaint.
Your entire example of a lab is plainly wrong, that's not a valid comparison as the circumstances are very differently and I don't see you whining about this kind stuff in any other series. Tell me what does your little lab tell you about eating a cursed fruit that makes you transform into light or ice or rubber or a hundred other things? What does your lab tell you about manifesting your soul into a sword? What does your lab tell you of being able to spit out 5 elements by having an entire non-existing energy system in your body? What does these series actually tell of all of this? You eat a devil's fruit, you get a power. Why? Because that's how it works. You become a Shinigami and you can get a fancy soul sword by training. Why? Because that's how it works. Chakra can pretty much do anything. Why? Because that's how it works. Kishi has always been going on and on about how Orochimaru preforms experiments and research, but in the end it that all came down to 'that's how it works'. Sharingan comes from the Rinnegan, that's just how it works. Like anyone knows what Orochimaru and Kabuto have been doing and what their research specifically entails.
And still despite all these things that make actually no sense whatsoever you only come whining about it with FT that actually bothered this time to provide a more than acceptable explanation. Hiro could have easily make Gajeel upgrade to steel without any reason. Him being iron and steel being an upgraded from of iron alone could have been used as an explanation. But then people complain it doesn't make sense. Now he gives a reasonable explanation and it's still not good enough.
Conclusion? You just want to point out for some reason how 'bad' this series is while not even having reasons for it and then you are the one who is going to throw around things about what makes sense and what not?
This is not an opinion what you have, it's an excuse to whine. It's the same thing as saying Christiano Ronaldo is a horrible football player. 'It's an opinion' but realistically that's just a shallow excuse to hate on him because for some reason you want to do that and you are doing the exact same thing with FT.