Says I have 2 segments:
segment1 segment2 in a transport zone
and have 2 Tier1 gateways:
segment1 connect to t1-gw1
segment2 connect to t1-gw2
how can I connect segment1 with segment2? which topology can I use? Is tier0 gateway needed?
Yes, the right way to do this is connecting them to a T0 gateway and it will route between the segments.
About the datapath it actually depends on how you have setup your T1s. If both your T1s do not have stateful services (no SR/no Edge configured), traffic will be fully distributed. However, if you have T1 SRs, traffic will go to your Edge nodes to go through the T1 SR. The way you need to think is that stateful services are realized on the gateway's uplink. So if you have traffic between T1s that have SRs, when going through the uplink traffic goes to the SR. T0 SR is not involved here.