FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article looks at the effect of BGP synchronisation rule on learning BGP routes from the BGP peer.  It basically states that unless and until a route is present in IGP table, the FortiGate will not advertise BGP route to the BGP peer. Nor will it install the BGP route received from the peer into the routing table. By default, the synchronization is disabled.

Configuration CLI

config router bgp
    set as 12849
    set router-id
    set synchronization enable
        config neighbor
            edit ""
                set remote-as 12849

Verification of configuration and troubleshooting

FGT # get router info bgp summary
BGP router identifier, local AS number 12849
BGP table version is 5
1 BGP AS-PATH entries
0 BGP community entries

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 4 12849 5075 2337 4 0 0 00:06:24 1

Total number of neighbors 1

FGT # get router info bgp neighbor received-routes
BGP table version is 13, local router ID is
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path
*>i0.0.0.0/0 100 0 i

Default route is being received from the peer. This route is not previously present in routing table. It is not in the IGP table.

FGT# get router info routing-table all
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
> - selected route, * - FIB route, p - stale info

The received route not present in the routing table.