Does Linux have a MBR?

Commonly, Linux is booted from a hard disk, where the Master Boot Record (MBR) contains the primary boot loader. The MBR is a 512-byte sector, located in the first sector on the disk (sector 1 of cylinder 0, head 0). After the MBR is loaded into RAM, the BIOS yields control to it.

Does Ubuntu use GPT or MBR?

If you boot (or dual-boot) Windows in EFI mode, using GPT is required (it’s a Windows limitation). IIRC, Ubuntu won’t install to an MBR disk in EFI mode, either, but you could probably convert partition table type and get it to boot after installing it.