It is expected that whenever the GRE TEP to the DC GW is added or deleted, then the below behavior is expected w.r.t the NAT :

1) When GRE TEP is deleted :

a) SNAT : If the corresponding DPN was the NAPT switch, then a new NAPT
switch is
re-elected among the other DPNs and rest of the non-NAPT switches point to
the new NAPT switch. The routes to the External Fixed IPs (SNAT IPs) are
updated with the new NAPT switch address.

b) DNAT : If the corresponding DPN was hosting VMs with the Floating IPs,
then
the routes to those FIPs are withdrawn from the BGP and removed from the
controller FIB.

2) When the GRE TEP is added :

a) SNAT : The correponding DPN will be delegated as the non NAPT switch
and made to point to the exsiting NAPT switch. Routes are not altered.

b) DNAT : If the corresponding DPN was hosting VMs with the Floating IPs,
then routes to those FIPs are advertised to the BGP and installed in the
controller FIB.