• Confirmed mycosis fungoides/Sezary syndrome, disease stage IB (defined as patches, plaque, or papules that involve 10% of the skin surface viscera) or higher.

• Age ≥ 18 years.

• ECOG Performance Score between 0-1

• Receipt of at least one prior systemic therapy for MF/SS.

• Previous systemic anti-cancer therapy must have been discontinued at least 2 weeks prior to treatment. See section 6.2 Subject Exclusion Criteria for guidelines regarding adjuvant and maintenance therapy for prior malignancy.

• Topical or systemic steroids (equivalent to ≤ 10 mg/day of prednisone) may be considered if dose has been constant and discontinuation may lead to rebound flare in disease, adrenal insufficiency, and/or unnecessary suffering.

• Prior therapy with gemcitabine allowed.

• Refer to Table 1 for laboratory inclusion criteria.

• The participant (or legally acceptable representative if applicable) provides written informed consent for the trial.

• A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies:

‣ Not a woman of childbearing potential (WOCBP) as defined in Appendix C

⁃ A WOCBP who agrees to follow the contraceptive guidance in Appendix C

• A male participant must agree to use a contraception as detailed in Appendix C of this protocol from screening and through 6 months after the last dose of gemcitabine or 120 days after the last dose of pembrolizumab (whichever is later) and refrain from donating sperm during this period.

∙ Table 1: Laboratory Parameters for Inclusion Criteria

∙ System : Laboratory Value

∙ Hematological Absolute neutrophil count (ANC): ≥ 1500/μL Platelets: ≥ 100 000/μL Hemoglobin: ≥ 9.0 g/dL or ≥ 5.6 mmol/L\^a

∙ Renal Creatinine OR Measured or calculated\^b creatinine clearance (GFR can also be used in place of creatinine or CrCl): ≤ 1.5 × ULN OR ≥ 30 mL/min for participant with creatinine levels \> 1.5 × institutional ULN

∙ Hepatic Total bilirubin: ≤ 1.5 ×ULN OR direct bilirubin ≤ ULN for participants with total bilirubin levels \>1.5 × ULN AST (SGOT) and ALT (SGPT): ≤ 2.5 × ULN (≤ 5 × ULN for participants with liver metastases)

∙ Coagulation International normalized ratio (INR) OR prothrombin time (PT) Activated partial thromboplastin time (aPTT): ≤ 1.5 × ULN unless participant is receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulants

∙ ALT (SGPT)=alanine aminotransferase (serum glutamic pyruvic transaminase); AST (SGOT)=aspartate aminotransferase (serum glutamic oxaloacetic transaminase); GFR=glomerular filtration rate; ULN=upper limit of normal.

• a: Criteria must be met without erythropoietin dependency and without packed red blood cell (pRBC) transfusion within last 2 weeks.

• b: Creatinine clearance (CrCl) should be calculated per institutional standard.

∙ Note: This table includes eligibility-defining laboratory value requirements for treatment; laboratory value requirements should be adapted according to local regulations and guidelines for the administration of specific chemotherapies.