Circuit diagram of an oscilloscope with 50 ohm load impedance
There are many sources on the subject, you need to search for something like "1 GHz PC scope". Many IC manufacturers offer amplifiers that can work only with 50-Ohm sources and loads. In fact, it is very difficult to get GHz-range with 1 MOhm input.

Here is an example, a reference design from Texas Instruments, "50-Ohm Oscilloscope Front-end Design Guide" . It looks like this:



All components are commercially available, and good luck.