A G-protein-coupled receptor (GPCR) is a cell-surface receptor that transmits ligand binding to a nearby heterotrimeric G protein composed of $\alpha$, $\beta$ and $\gamma$ subunits.

In the resting state, the $G\alpha$ subunit binds GDP and associates with $G\beta\gamma$.

Receptor activation promotes nucleotide exchange

Ligand binding stabilizes an active GPCR conformation. The activated receptor interacts with the G protein and promotes GDP release from $G\alpha$. GTP then binds.

The sequence is

ligand binds GPCR
      ↓
active GPCR binds G protein
      ↓
Gα releases GDP and binds GTP
      ↓
Gα-GTP and/or Gβγ regulate downstream targets

The receptor therefore functions as a guanine-nucleotide exchange catalyst for the heterotrimeric G protein.

Both branches can signal

Activation changes the interactions among $G\alpha$, $G\beta\gamma$, the receptor and downstream effectors. Depending on the G-protein subtype and cell, $G\alpha$-GTP and $G\beta\gamma$ can regulate enzymes or ion channels.

It is therefore too simple to say that only the $\alpha$ subunit 'carries the signal.'

GTP hydrolysis resets the cycle

$G\alpha$ has GTPase activity. Hydrolysis

$$\mathrm{GTP\rightarrow GDP+P_i}$$

returns it toward the GDP-bound state, allowing the resting heterotrimer to reform.

Regulatory proteins can accelerate this hydrolysis and therefore shorten signaling duration.

A GPCR pathway is thus a coupling device:

$$\boxed{\text{ligand-bound receptor}\rightarrow\text{GDP/GTP molecular switch}\rightarrow\text{effector activity}}.$$

Different GPCRs connect to different heterotrimeric G proteins and effectors, which lets the same receptor architecture control many physiological processes.