Over a p p -adic local field F F of characteristic zero, we develop a new type of harmonic analysis on an extended symplectic group G = G m × S p 2 n G={\mathbb {G}}_m\times {\mathrm {Sp}}_{2n} . It is associated to the Langlands γ \gamma -functions attached to any irreducible admissible representations χ ⊗ π \chi \otimes \pi of G ( F ) G(F) and the standard representation ρ \rho of the dual group G ∨ ( C ) G^\vee ({\mathbb {C}}) , and confirms a series of the conjectures in the local theory of the Braverman-Kazhdan proposal (Braverman and Kazhdan, 2000) for the case under consideration. Meanwhile, we develop a new type of harmonic analysis on G L 1 ( F ) {\mathrm {GL}}_1(F) , which is associated to a γ \gamma -function β ψ ( χ s ) \beta _\psi (\chi _s) (a product of n + 1 n+1 certain abelian γ \gamma -functions). Our work on G L 1 ( F ) {\mathrm {GL}}_1(F) plays an indispensable role in the development of our work on G ( F ) G(F) . These two types of harmonic analyses both specialize to the well-known local theory developed in Tate’s thesis (Tate, 1950) when n = 0 n=0 . The approach is to use the compactification of S p 2 n {\mathrm {Sp}}_{2n} in the Grassmannian variety of S p 4 n {\mathrm {Sp}}_{4n} , with which we are able to utilize the well developed local theory of Piatetski-Shapiro and Rallis (1986) and many other works) on the doubling local zeta integrals for the standard L L -functions of S p 2 n {\mathrm {Sp}}_{2n} . The method can be viewed as an extension of the work of Godement-Jacquet (1972) for the standard L L -function of G L n {\mathrm {GL}}_n and is expected to work for all classical groups. We will consider the Archimedean local theory and the global theory in our future work.