A Study to Asses Safety and Tolerability of ITI-1284 Given Alone or as an Additional Treatment in Participants With Generalized Anxiety Disorder (GAD)

Inclusion criteria for Roll-over participants:

* Provide written informed consent before the initiation of any study specific procedures
* Is currently an outpatient, and is anticipated to maintain outpatient status for the duration of the study
* Male or female of childbearing potential and agrees to use a highly effective method of birth control from the time informed consent is provided until 30 days after the last dose of ITI-1284 for females and 90 days after the last dose of ITI-1284 for males
* Ability to follow study instructions and likely to complete all required visits
* In the opinion of the Investigator, the patient must have safely completed the lead-in study (ITI-1284-301 or ITI-1284-302)
* If entering from lead-in study ITI-1284-301, must be expected to continue their ongoing anti-depressant therapy (ADT) or buspirone from the lead-in study

Inclusion criteria for De-novo participants:

* Provide written informed consent before the initiation of any study-specific procedures
* Male or female participants greater than or equal to (\>=) 18 years of age
* Has a body mass index (BMI) of 19-40 kilogram per meter squared (kg/m\^2), inclusive
* At screening, meet diagnostic and statistical manual of mental disorders, fifth edition, text revision (DSM-5-TR) diagnostic criteria for moderate or severe generalized anxiety disorder (GAD) (current GAD) as confirmed by the investigator or sponsor-approved rater using the structured clinical interview for DSM-5, clinical trials version (SCID 5-CT)
* Is currently an outpatient, and is anticipated to maintain outpatient status for the duration of the study

Exclusion criteria for Roll-over participants

* In the opinion of the Investigator, the participant has a significant risk for suicidal behavior during the course of her/his participation in the study or is considered to be an imminent danger to her/himself or others, and/or: a. At baseline, the participant scores "yes" on suicidal Ideation Items 4 or 5 of the Columbia-suicide severity rating scale (C-SSRS) "Since Last Visit" version; b. At baseline, the participant scores \>= 5 on the Montgomery-Asberg depression rating scale (MADRS) Item 10 (suicidal Thoughts); c. The participant is considered to be an imminent danger to themself or others based on the assessment of the Investigator
* The participant is breast-feeding or pregnant; female participants of childbearing potential must have a negative urine pregnancy test at baseline
* Based on the Investigator's clinical judgement, any abnormal clinical laboratory test or electrocardiogram (ECG) results obtained throughout the lead-in study that are considered clinically significant and preclude safe participation in the study
* The participant has any other condition that may be detrimental to the participant if they participate in the study
* In the opinion of the Investigator, the participant is unable to comply with study procedures or judged to be inappropriate for the study

Exclusion criteria for De-novo participants

* Within the participant's lifetime, has one of the following confirmed DSM-5-TR psychiatric diagnoses: a. schizophrenia, schizoaffective disorder, schizophreniform disorder or other psychotic disorder; b. bipolar disorder
* Within 6 months of screening, has a confirmed DSM-5-TR psychiatric diagnosis other than GAD, including: a. other anxiety disorders; b. moderate or severe alcohol or substance use disorders; c. moderate or severe major depressive disorder (MDD)
* MADRS total score greater than (\>) 18 at screening or baseline
* The participant has received electroconvulsive therapy (ECT) or vagal nerve stimulation within the past 5 years, or repetitive trans cranial magnetic stimulation within the last 2 years, or had a failure in response to ECT at any time
* The participant has known hypersensitivity or intolerance to ITI-007 (lumateperone), ITI-1284, or to any of the excipients