From what I understand all Thai girlfriends like a gift they can show their friends and something with real value.
For a Thai lady to have a gift that can be pawned in on a rainy day is important.
Gold will always bring a smile to your Thai girlfriends face, either a necklace or a bracelet. Make sure it is at least 2 Baht in weight, which in todays money would cost about 26,000 Baht. That I promise you will raise a smile on her pretty face.
The other gift a Thai girlfriend is looking for is marriage. Why not give her the ultimate mate gift and marry your Thai girlfriend. Then all you need is the marriage visa, and you can give her the gift of living with you as man and wife for the rest of your lives.