Found out/announced via RockStar Support, PC Gamers will be able to start pre-loading GTA V on the 7th of April, 2015.
It’s worth noting that the pre-load is confirmed to be viable for those who bought the game on either Rockstar Warehouse or Steam, and if you bought it somewhere else, you would have to contact and see if a pre-load is possible.
This is what was asked on the Rockstar Support:
Question: I pre-ordered the downloadable version of GTAV for PC. Will pre-loading be supported?
Answer: Yes, pre-loads via Rockstar Warehouse and Steam will be available on April 7th. We will update this page with an exact time as soon as that information is available. If you pre-ordered GTAV from a digital retailer other than Rockstar Warehouse or Steam, please contact your retailer’s customer support departments for details on pre-loads.