Sufyan al-Thawri (ra) said, gratitude to Allah is avoiding what Allah forbade.
Some said that it is thanking Allah in good times and that patience with trials is Zuhd.
Some of them said that blessings continue as long as you thank Allah and they disappear when you are ungrateful.
Umar bin abdul Aziz said, remembering blessings is considered as being grateful for them.
Be grateful to the King with your tongue and your acts of worship and avoid what He has forbidden so that we may be grateful to Him.