Can I receive BTG to my BTC address after BTG hardfork? I read that on 25. oct. 2017. the BTG hardfork will happen and every one holding BTC on their address will receive BTG for their BTC in 1:1 ratio.
What happens if I have X amount of BTC on my address A, and after the hardfork happens I still receive BTC to that address. 

Can I change that BTC to BTG in 1:1 ratio even some time after the hardfork happened?

For example if I have 1BTC on my address, then at the time of the hardfork it will be converted to 1BTG. Then let say, weeks later I receive 1BTC to the same address. 
Can I spend 2BTGs then?
No

Bitcoin Gold has replay protection.  After the fork, all further transactions on the BTC chain will not transfer BTG, nor vice versa.  

In your example, when someone sends you 1 BTC after the fork, the corresponding 1 BTG will still belong to the sender, not to you.  You will not gain any BTG from the transaction.  (Of course, they could send it to you separately if they wanted.)

It will also not be possible to spend BTC as BTG or vice versa.  To exchange one for the other, you will have to find someone to make a trade with, and agree on an exchange rate, which will probably not be 1:1. (You can use an organized exchange to make this efficient, but it's the same thing in principle.)