The easiest way would be to set Rpi's IP address as default gateway on PC. If PC is getting it's network settings via DHCP server, you'll have to create static DHCP lease for that PC. On that particular lease you'll set different IP address as gateway. (It will be some more work than I just wrote, but should direct you towards your goal).
If, OTOH, PC is set up statically, then just change settings on the PC.

It's not exactly trivial to enforce special config to particular host. Perhaps the easist way would be using DHCP options ... create new one with code=3 (that's default gateway), enter the value (no idea how it should look like) and assign this option to static DHCP lease. Hopefully explicit DHCP option will overwrite the one from generic DHCP network settings ...

I hope somebody more knowledgeable about DHCP options will pop up ...

Relevant RFC 2132 defines option format to be array of octets and first octet (after the DHCP option number) should be the length of the array (and must be multiple of 4). So I assume that when defining option value should be entered as HEX number, e.g. 0x04c0a80a0d ... I've no idea what should be the order of address octets (MSB or LSB) nor do I gave idea whether the previous value would translate to correct array of octets.

Anyway, when you're testing if the setting works, you should appply the change on RB, then disconnect and reconnect ethernet cable of the PC under test (I'm assuming wired connection; disable and re-enable wireless interface if you're using wireless connection) - keep it disconnected for at least 10 seconds, then wait for a few seconds for DHCP protocol to do the magic and then check the IP settings - router address in particular. If it changes from whatever is default setting on your DHCP server, then we're on right track and it remains to discover the right value format. If it doesnt, then we'll have to think of something else.