What should I do with the profits I received from the gold trading company after I requested to withdraw my money and the company refused to do so before the contract period ended, and am I sinful for investing in it?

So long as you cannot فسخ (dissolve) the contract now due to the company's refusal, there is no sin upon you for letting the matter remain until the term ends, then you can terminate this partnership. The actual profits from your money are lawful for you, whether the partnership was invalid or valid. Sin is lifted from the ignorant person who did not neglect to ask.