Yes, in most cases you can still get your gym added. If your gym is not in KAYA's database yet, fill out the Add Gym Request form with the gym's name, full address, and website. The KAYA team typically adds new gyms within 10 to 14 business days.
One thing worth knowing: being added to KAYA's database only gives you access to logging climbs through the grade list feature. The full interactive guidebook experience (with setter names, routes, and gym specific features) is only available for gyms that have partnered with KAYA directly. If you want your gym to have the full guidebook experience, ask gym staff to reach out to the KAYA team.