I'm sorry you are so worried about your sweet Winnie. I can tell you love her dearly.
I don't know anything about the GGT level, but have experienced elevated ALKP and ALT levels in two of my Scotties. First, Scottish Terriers often run an elevated Alkaline Phosphatase level.
My rescue dog Tipper had elevated ALKP from age 4. We tried her on a supplement called Denosyl for several months. The levels dropped but never reached the normal range. We also tested her for Cushings Disease, but she exhibited several additional symptoms. At age 10, she had surgery to remove her spleen which compromised her immune system. At age 11 she needed bladder surgery and a liver biopsy at the same time discovered she had bacterial hepatitis. Her two liver values were through the roof (ALKP 2,000, ALT 1,800). She was started on antibiotics and a liver supplement called Denamarin (basically milk thistle and SAM-e.) We retested in 6 weeks and the values each dropped by around 800 points. A health issue not related to the liver occurred and she passed away so I don't have long-term results.
My first Scottie, Mac had elevated ALKP levels discovered around age 7 with a senior blood panel. We tried the Denosyl on him as well, but never got the value in the normal range (his level was typically 600-800). Mac did pass from liver failure/suspected liver cancer at age 14 1/2, which isn't bad for a breed with a life-expectancy of 12-15 years. My vet felt the liver problem was a secondary tumor/cancer and I didn't pursue any testing to discover where the cancer may have originated. He lived a wonderful life for many years with his ALKP elevated.
Both the supplements worked well for my dogs. I will say that the Denamarin seems a better choice. I will link to both drugs so you can review and see if this is something you want to talk to your vet about
We only had the biopsy for Tipper due to an unrelated abdominal surgery. Mac's problem was discovered on ultrasound. Your vet can do an ultrasound of the abdomen to check for tumors in the liver. How to pursue depends on how aggressively you wish to treat. The supplements should be a safe thing to give if you want to check into one of them.