Turtle vents on house with Gable vents?

My roof is getting replaced and have been told allot of different things by different contractors, some recommend a ridge vent but I don't have soffit vents so ive heard that's a no no. I cant seem to find any information out about whether its a good idea to use turtle vents on a house with gable vents on both ends though so would love some advice. Currently I have a gable vent at either end and no other venting.