There is no "Gallery" app. You can use the Google Photos app to browse and manage your photos on the device. To find them on your computer, simply plug your phone into your computer via the USB-A to USB-C or the OTG dongle, set your phone on file transfer mode, as opposed to charging mode and then open a file explorer window. Once you find and open the device, you'll see the Internal Storage folder. Inside it, you'll find a DCIM folder for pictures/videos taken with the Pixel, and a Pictures folder which will contain other folders for images saved onto the device by various apps (you should see a folder per app that has that apps name as the folder name).
I hope this helps!