I've seen modified garter st used for just what you're looking for, straps on a tank or camisole. It's used over 6 sts as follows:
Row 1: (RS)Knit.
Row 2: (WS)Knit.
Row 3 : K1, slip 1, k2, slip 1, k1.
Row 4: Knit.
I think straight garter st is the usual, knit all the rows and 'straight' is used to distinguish it from the modified garter st.