This study is being done to answer the following questions:

Can we lower the chance of your gastric cancer from growing or spreading by administering paclitaxel chemotherapy directly into your abdominal cavity in addition to chemotherapy given through a vein in your arm? Will administering paclitaxel chemotherapy directly into your abdominal cavity, in addition to chemotherapy given through a vein in your arm help you live longer? We are doing this study because we want to find out if this approach is better or worse than the usual approach for your gastric cancer. The usual approach is defined as care most people get for gastric cancer.

If you decide to take part in this study, you will first receive a surgical procedure called a diagnostic laparoscopy. This will help the study doctors learn more about your gastric cancer. Laparoscopy is a minimally invasive surgery for which you will be placed under general anesthesia. Then the surgeon will make small incisions (5mm) on your belly through which a camera and thin instruments are introduced to evaluate the abdomen. This procedure takes about 1 hour to complete. Your study group will be assigned during the surgery. The study groups are described further in the 'What are the study groups?' section below.

If you are placed into the study group 1, you will not have an intraperitoneal port (a small device which is placed under the skin and fat of your upper abdomen and a tube that is placed into the abdomen).

If you are placed into the study group 2, you will have an intraperitoneal port placed. The reason is that in addition to standard chemotherapy, which is given through a vein in your arm, this port will be used to deliver the medication paclitaxel directly inside your abdomen when you are ready to start study treatment.

It is important to know that you will not know your study group until after the surgery is over. This is because information that is learned during the surgery will help determine which study group you are put in.

Once you have fully healed from this surgery, you will start study treatment. Depending on which study group you are assigned, you will either receive a standard chemotherapy regimen (the regimen will be chosen by you and your doctor) if you are in study group 1, or paclitaxel through a tube in your belly plus chemotherapy given through a vein in your arm if you are in study group 2. All participants will get treatment for three (3) months after which you will undergo reevaluation. If the disease is under control or responding to treatment, you may continue the assigned treatment until your disease gets worse, the side effects become too severe, or you may be offered a surgical procedure to remove the cancer if the amount of disease is low and can be completely removed as determined by a surgeon.

There is a very small chance that during the laparoscopy surgical procedure, the doctor might find something called "intra-abdominal adhesions". These are areas where the stomach has healed previously and created scar tissue. If this scar tissue prevents the surgeon from being able to place a port in the correct area, you would be ineligible to receive the study treatment. If this happens, you may still receive standard of care therapy after your surgery, but you will not be able to continue on the study. If you have more questions about this, you can ask your surgeon or the study team to help.

After you finish your study treatment, your doctor or study team will watch you for side effects. They will continue to follow your condition every three (3) months during the first two (2) years, then every six (6) months until year 5. You may be reevaluated with Chest/Abdomen/Pelvis scans every three-six (3-6) months for up to five (5) years if decided by your doctor.

This study is researching an investigational drug called marlotamig (REGN7075) by itself and in combination with cemiplimab with or without chemotherapy. The study is focused on patients with certain solid tumors that are in an advanced stage.

The aim of the study is to see how safe and tolerable marlotamig is by itself and in combination with cemiplimab (with or without chemotherapy), and to find out what is the best dose of marlotamig to be given to patients with advanced solid tumors when combined with cemiplimab (with or without chemotherapy). Another aim of the study is to see how effective marlotamig by itself, or in combination with cemiplimab (with or without chemotherapy), is at treating cancer patients.

The study is also looking at:

* Side effects that may be experienced by people taking marlotamig by itself and in combination with cemiplimab with or without chemotherapy
* How marlotamig works in the body by itself and in combination with cemiplimab with or without chemotherapy
* How much marlotamig is present in the blood when given by itself and in combination with cemiplimab with or without chemotherapy
* To see if marlotamig by itself and in combination with cemiplimab with or without chemotherapy works to treat cancer by controlling the proliferation of tumor cells to shrink the tumor
* Whether the body makes antibodies against the study drugs (marlotamig and cemiplimab) (which could make the drug less effective or could lead to side effects)