Dense β-Si4Al2O2N6 and β-Si4Al2O2N6-0.5SiO2 ceramics were obtained from α-Si3N4, α-Al2O3, AlN, and Y2O3 upon sintering green bodies consolidated by aqueous gel casting. For comparison purposes, a β-Si4Al2O2N6 was also prepared by the conventional dry-powder processing route. In the case of gel-cast β-Si4Al2O2N6, the as-purchased AlN powder was treated with H3PO4 and Al(H2PO4)3 prior to use along with α-Si3N4, α-Al2O3, and Y2O3. The gel-cast β-Si4Al2O2N6 exhibited superior hardness (1423 ± 6 Hv), fracture toughness (3.95 ± 0.3 MPa⋅m1/2), and coefficient of thermal expansion (CTE) (3.798 × 10−6/°C between 30 and 1000 °C) in comparison to the ceramic consolidated by conventional dry pressing, which exhibited only 1317 ± 5 Hv, 3.30 ± 0.2 MPa⋅m1/2, and 3.532 × 10−6/°C between 30 and 700 °C. The in situ-generated ∼9 wt% SiO2 has considerably reduced the dielectric constant and CTE of β-Si4Al2O2N6 from 7.30 to 6.32 and from 3.798 × 10−6/°C to 3.519 × 10−6/°C, respectively. The loss tangent property of the investigated materials was little influenced by the variation of chemical composition and processing route.