@yacud With failover and multiple tiers, it will use the Tier1 gateways until it meets the criteria of a failure (specified packet loss or latency).
Then it will route all traffic on the Tier2 gateway until Tier1 gateway is back within acceptable limits.
If you want to load balance you could set multiple gateways as Tier1 and it will split traffic between them, you can set a "weight" in the gateway options to have it balance the traffic unevenly (e.g. put 2x as much on WAN1 vs WAN2)
As far as I know, there is no way for it to know what the maximum throughput of your link is - just trying to split it evenly if you want load balancing.