what should be used to connect to local networks (for example 10.20.100.*
with NAT router address 1.2.3.4 and 10.20.200.* with NAT router address
2.3.4.5)?

NAT router runs netbsd.

is gre and gif doing the same thing, but with different protocol?

what is more effective/fast and can encryption be used on them (gif manual
points to IPsec but i can't find anything more).

have anyone used vtund with tun interface with encrypting and/or
compression?

thank you for answer i will need such thing