A Clinical Study to Test How Effective and Safe GLPG1205 is for Participants With Idiopathic Pulmonary Fibrosis (IPF)

Inclusion criteria: Participants who meet all of the following criteria are eligible for the study: * A diagnosis of IPF within 5 years prior to the screening visit as per applicable American Thoracic Society (ATS)/European Respiratory Society(ERS)/Japanese Respiratory Society (JRS)/Latin American Thoracic Association (ALAT) guideline. Participants receiving local standard of care for the treatment of IPF, defined as either pirfenidone or nintedanib at a stable dose for at least 8 weeks before screening, and during screening; or neither pirfenidone nor nintedanib (for any reason). A stable dose is defined as the highest tolerated dose. Prednisone at steady dose less than or equal to 10 mg/day or equivalent glucocorticoid dose is allowed (stabilized 4 weeks prior to screening and continued without variation of dose or regimen). Supportive care like supplemental oxygen or pulmonary rehabilitation is allowed. * Meeting all of the following criteria at screening and during the screening period: * Forced vital capacity (FVC) greater than or equal to 50% predicted of normal * Disease progression, defined as a decline of FVC (% predicted or milliliters \[mL\]) at the investigator's discretion, during the 9 months prior to the screening period and confirmed at the screening visit * Diffusing capacity for the lungs for carbon monoxide (DLCO) greater than or equal to 30% predicted of normal (corrected for hemoglobin) * Ratio of forced expiratory volume in one second (FEV1) to FVC greater than or equal to 0.70 * In a stable condition and suitable for study participation based on the results of a medical history, physical examination, vital signs, 12-lead electrocardiogram (ECG), and laboratory evaluation. Stable condition is based on the clinical judgment of the investigator, co-morbidities should be treated according to the local applicable guidelines. Concomitant medication for chronic comorbidities should be stabilized from 4 weeks before screening and during the screening period (stable defined as no clinically relevant change according to the investigator's judgement). * Able to walk at least 150 meters during the 6 Minute Walk Test (6MWT) at screening; without having a contraindication to perform the 6MWT. This list only describes the key inclusion criteria. Exclusion criteria: Participants meeting one or more of the following criteria cannot be selected for this study: * Known hypersensitivity to any of the investigational medicinal product (IMP) ingredients or a history of a significant allergic reaction to any drug as determined by the investigator (e.g. anaphylaxis requiring hospitalization). * Current immunosuppressive condition (e.g. human immunodeficiency virus \[HIV\] infection, congenital, acquired, medication induced, organ transplantation). * Positive blood testing for hepatitis B surface antigen (HBS Ag) or hepatitis C virus (HCV) antibody (if positive, confirmed by HCV ribonucleic acid \[RNA\] positivity). Note: Participants with a resolved hepatitis A at least 3 months prior to first dosing of the IMP can be included. * History of malignancy within the past 5 years (except for carcinoma in situ of the uterine cervix, basal cell carcinoma of the skin that has been treated with no evidence of recurrence, and prostate cancer medically managed through active surveillance or watchful waiting, and squamous cell carcinoma of the skin if fully resected). * Acute IPF exacerbation within 3 months prior to screening and during the screening period. * Lower respiratory tract infection requiring antibiotics within 4 weeks prior to screening and/or during the screening period. * Interstitial lung disease associated with known primary diseases (e.g. sarcoidosis, amyloidosis), exposures (e.g. radiation, silica, asbestos, coal dust), and drugs (e.g. amiodarone). * History of lung volume reduction surgery or lung transplant. * Unstable cardiovascular, pulmonary (other than IPF) or other disease within 6 months prior to screening or during the screening period (e.g. coronary heart disease, heart failure, stroke). * Participant participating in a drug, device or biologic investigational research study, concurrently with the current study, within 12 weeks or 5-half-lives of the agent, whichever is longer, prior to screening, or prior participation in an investigational drug antibody/biologic study within 6 months prior to screening. This list only describes the key exclusion criteria.