Call forwarding via the Internet using a SIP
URI .
eg SIP:123456@eu.voxalot.com
If Sipgate have a way of setting up call forwarding to a URI as per the example, then that is what you want to do. Some providers only let you forward to a real number, then they charge you for the call to that number. You don't want to forward to a number, but rather to a SIP URI as above.
I don't know if Sipgate offer SIP URI forwarding, but it appears they don't....