Is it worth buying a Gibson Sgj instead of a gibson sg standard? If it is so, Is it also possible to upgrade a Gibson SGJ in order to make it sound just like a gibson SGstandard ?? Please reply if you do know because I am currently stuck between choosing either a gibson SGJ or a standard Fender Stratocaster . I am looking for a versatile guitar which has a clean tone as well as a insane metal tone.
Not sure this is the correct place for you to post this but I will answer regardless. I can't comment on the Gibson SG questions but I own a MIA HSS Strat, and I believe it is the most versatile guitar I have ever come across. It is a high end model, but I think the ability to have a humbucker in the bridge position and that single coil neck/mid position open up A LOT of versatility. Having said that I ust skimmed SG standard prices and that was around the same as my Strat. For that money you have many options, many of them good.
The thing with Gibson is you have to try them. The QC is such that you can get a terrible Standard or a fantastic low end model.

Check the specs of the exact model of SGJ you're looking at, a lot of them have the same pickups as the more expensive models.

And as for the choice between an SG or a Strat....only you can decide that. They're so different it comes down to personal preference & we can't help you with that.
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It would be possible to come close with some extensive work replacing electronics, but you'll find the maple neck of the SGJ won't ever sound quite the 'exact' same as the mahogany neck of the Standard.

The SGJ itself is a good guitar for the price. I dang near bought one this year, but ended up with a Melody Maker instead... couldn't pass up the dual P-90 pickups. If you spent the additional cash to upgrade the hardware and electronics to 'Standard' specs, it'll still be an SGJ if you ever went to sell it off. Likewise, my MM is a great guitar, but it will always be a MM in it's resale value.

Go to a shop, plug an SGJ in and see how it plays vs the Strat. They are going to feel and play very differently due to construction and overall shape. Tone of the humbuckers in the SGJ are going to be massively different from the quack the Strat should give. Both are fine guitars in their own right, but certainly vastly different.