Resident Evil Gaiden was originally supposed to be canon and coincide with Resident Evil Code: Veronica but this plot point was later dropped in future titles. Gaiden was referenced in Wesker's Report, specifically mentioning Leon joining an anti-Umbrella organization. However, the end of Gaiden implies that the "Leon" you see is actually a Bio Organic Weapon impersonating Leon. Resident Evil 4 makes it clear that this is not the case though, and the game has since been stricken from the official series timeline, rendering Gaiden a fully non-canon entry in the series.