First, the reference to 2.5mm that you heard means that the chainline on GRX is 2.5mm further out than Shimano’s road chainline. This can be seen on Shimano’s site. The GRX chainline is 47.0mm, measured from the centerline of the bike to the center of the two chainrings. The chainline for Shimano’s current road groups is 43.5mm.
(As a side note, the 47.0mm GRX chainline minus the 43.5mm road chainline is a 3.5mm difference. The interviews and reviews all did say the chainline was moved by 2.5mm. I don’t know why there is this discrepancy.)
You may be able to get away with using a standard Shimano road crank, although the shifting might be impaired and you probably won’t have the gear you want (you’d be limited to a 50/34 gear). For a different replacement crankset, you’d want to find a crank with a chainline of 45.0 to 47.0mm - or in theory something a bit wider than 47.0 might be fine also, but I don’t think any road or gravel cranks with wider chain lines exist.
Full Speed Ahead has one option that's relatively inexpensive that has a 46mm chainline. This isn't an exhaustive list of potentially compatible cranks. White Industries’ R30 crankset lists a 2x chainline of 45mm and says that Shimano derailleurs, including GRX, work best with it. However, this is a niche US manufacturer and it may be hard to get in Europe. However, this is why I think a 45.0mm chainline should work OK with GRX. Rotor is a Spanish company that makes modular cranksets, and they have an option for a 46mm chainline - note that they are quite pricey because they're a small operation.
(Sram’s current Force and Rival Wide cranks have 47.5mm chainline. Their road cranks have 45.0mm chainlines. Those chainlines would work, but do note that their 12s AXS groupsets use a different roller diameter, and thus they won't work well for Shimano chains. The previous version of the answer neglected to mention this because I forgot.)
Cranks are very seldom sold as right arm only or left arm only. They are sold as a pair of arms. In English, the word ”crankset” always means both arms. Here is one English advertisement for a GRX left arm only.