Where can we buy the U74 with the GPU for our own boards?
The FU740-C000 was only manufactured for the SiFive Unmatched boards. It was never sold separately. SiFive is not in the business of manufacturing and selling chips. Their business is licensing cores. As far as I know, the only chip they ever sold directly was the FE310 as used on the original HiFive1. If you want to license a core and fab chips you can contact SiFive sales. If you wait until Summer 2023 you might be able to buy Horse Creek P550 parts from Intel and/or SiFive. They will sell boards. I don’t know if they will sell the chips separately though.
Otherwise, there are parts available from other suppliers that have SiFive cores on them, like the MicroChip PolarFire which is U54 based with a FPGA on chip intended for embedded linux systems. Another choice is the StarFive JH7110 which will be on the new Pine RISC-V based board and is U74 based with an Imagination Vulkan GPU.
There are also non SiFive cores available, but this is the wrong place to discuss them.
Will Horse Creek P550 have a GPU?
Full details for the Horse Creek part haven’t been released yet, but I would guess no. The FU740 part didn’t have a GPU. It had a PCIe root and the board had a slot for a video card. I would guess that Horse Creek will be similar. Note that this part is primarily for software development, and for testing a new Intel process. Adding a GPU is extra work that likely wasn’t in the plans. There may be later parts, possibly from other vendors, that add a GPU to the P55 core.