I have started Appian by start-suite.sh , checked the status using checkengine.s

Hi Eduardo, We got a fresh installation of both Appian and Oracle DB and migrated everything. When we started the server we got the same issue saying "Inconsistent DBID on GW1. Server Database ID: 4, Gateway Database ID: 0N" for all the gateways except process-analytics. Does the fresh installation of Oracle DB making an issue?

No, the Oracle DB doesn't make any difference but this could be the result of an incorrect migration in the sense that the engines were not in sync before starting up the environment. You can use the steps described here to re-sync and keep an eye on the environment to make sure this doesn't happen again (assuming you have a stable network connection between both servers).

For more information please take a look at this section of the documentation forum.appian.com/.../High_Availability_and_Disaster_Recovery_Configurations this section has been recently updated so you may find useful to read it

We are on Appian version 6.5.1. Is it possible to migrate data from Version 6.5.1 to 7.2?

Yes, the steps are described in the migration guide forum.appian.com/.../Migration_Guide