Both churches are what they call "Sabbath keepers". What are the main differences in doctrine between the Seventh Day Adventist Church and the Worldwide Church of God, now known as Grace Communion International?
It is true that both of these churches observe the seventh-day Sabbath, Saturday. Some of the distinctions beyond this, however, include some key doctrinal points. Note that this is not a full list, and I am answering with respect to the "Seventh-day Church of God" (as it seems to translate best from the local language where I am--I presume this to be the local equivalent of the Worldwide Church of God of this question) with considerable input from the "Church of God: A Worldwide Association, Inc." website.
|Seventh-day Adventists||Church of God: A Worldwide Association|
|Officially believe in a Trinity consisting of God the Father, God the Son, and God the Holy Spirit--three co-equal, co-eternal beings; many members do not accept this, some have been disfellowshipped over disagreements on the Godhead||Do not believe in a Trinity, but believe in a "Family" of God consisting of the Father and the Son, both of whom have eternally existed, but the spirit is not a separate entity, or being (Binitarian)|
|Follow a health message, including the avoidance of unclean meats like pork, seafoods, etc. in accordance with Leviticus 11 and Deuteronomy 14||Officially follow the dietary guidelines of Leviticus 11 and Deuteronomy 14; Some congregations do not teach any dietary restrictions|
|Do not drink or smoke||Social drinking is allowed|
|Observe a "communion service" including footwashing and the partaking of the emblems of "wine" (non-alcoholic grape juice) and unleavened bread (crackers) once quarterly||Observe a "Passover"—foot-washing, unleavened bread and wine—are part of this service, once annually to commemorate Jesus' death|
|Sabbath is the seventh-day of the week, Saturday, and is observed from Friday evening at sunset to Saturday evening at sunset||Sabbath is the seventh-day of the week, Saturday, and is observed from Friday evening at sunset to Saturday evening at sunset|
|The feasts of the Old Testament are part of the "ordinances" that were nailed to the cross, and should no longer be followed (to follow them would indicate disbelief in Christ's atonement having fulfilled these types); the feasts are replaced by the communion service (last supper ceremony) and by campmeetings and other general assemblies of the church||The feasts are still binding and should be observed today, they contain lessons of salvation and will be observed even through the Millennium|
|Tithing is practiced in which 10% of increase is given to the church, to be used for the support of the ministry; Adventists have a stable system in which these tithes are centralized and ministers/pastors receive a steady monthly salary from the local conference; other expenses, such as church operating budget, maintenance, etc. are supported by freewill offerings, not tithes||Tithing is practiced and the funds go to assist in preaching the gospel, church maintenance, and care for needy members; a special tithe is collected at feast observances|
|Military service is at the discretion of individual members, but a status of non-combatant ("conscientious objector") is encouraged; as such, many join the medical corps to assist with healing services for those injured in battle; Desmond Doss, a congressional medal-of-honor recipient in the U.S. and the inspiration for the movie "Hacksaw Ridge," was Seventh-day Adventist||Christians should not serve in the military; even hating one's brother is tantamount to murder, and soldiers will have Sabbath problems, etc.; we should obey God rather than man--difficult to do when one must obey one's commanding officer|
|At the end of about 6000 years Jesus will return and take his people to heaven for 1000 years (the millennium), after which they will return to earth, with the New Jerusalem, to establish Jesus' kingdom on the earth made new (following the final destruction of the resurrected wicked)||At the end of "this present evil age," Jesus will return and establish his kingdom forever, beginning with a 1000-year period commonly known as the millennium; following this the second resurrection takes place and people have one last opportunity to become converted; the incorrigible will be raised in the third resurrection to receive their part in the lake of fire for a brief existence--and no further resurrection is to take place|
|Seventh-day Adventists study carefully and teach the great prophecies of the Bible, including those of Daniel and Revelation||Do not emphasize prophetic fulfillments|
Again, this list does not propose to be complete--a complete comparison might require vastly more space (a book?). This highlights some of the more noticeable features from my perspective. Further, there seems to be a great consistency, worldwide, to Seventh-day Adventist beliefs and practices, whereas the "Worldwide" (Seventh-day) Church of God seems to have more regional variation, and the stated beliefs per the website may not accurately reflect the experience of a given member in an individual congregation.
Disclaimer: I do not presume to be fully accurate with this, particularly with the lesser understood (for me) second column in the table. If you have points to add, please share them in the comments and I may be able to update the information in the table. I am considerably more confident of the factual accuracy of the first column (left side of table).