I have been approached by an investor saying that he wants to wire money to my account as a "gift" to avoid taxes. I would get to keep 5%. He has shown many documents showing proof and successful transactions. Is this a scam?
Yes, it's a scam.
At best you will lose money, for example he's going to claim he needs your bank account login info to validate that the transaction has arrived and will then clear you out, or he's going to send you money via a reversible route and require you to send 95% of the money on via a non-reversible route, after which the original transfer will be reversed and you'll be left having sent money you can't get back).
At worst he's money laundering (the money he sends you comes from crime, directly or indirectly and he needs it to go through a "clean" account) which would leave you liable for criminal charges.
A genuine investor would not approach a stranger online. Why should he trust you?!
Do not engage further; delete the messages, block him and move on.