Place jicama, cucumber, red onion, and bell peppers together in a mixing bowl. Toss with lime juice and avocado oil until combined; refrigerate until chilled, about 20 minutes.
Season the mashed avocado with garlic salt. Spread the avocado mixture evenly over tostada shells. Top evenly with jicama mixture and grape tomatoes. Garnish with cilantro and serve with lime wedges.