Here are five possible code completions for the given function:

**1. Checking the length of the received data**
```c
static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
{
    unsigned int len;
    len = gsm->rx_len;
    if (len > 0 && len <= GSM_MAX_RX_LEN) {
        // Process the received data
        gsm->rx_buf[len] = c;
        gsm->rx_len++;
    } else {
        // Handle error or overflow
    }
}
```

**2. Verifying the received character**
```c
static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
{
    unsigned int len;
    len = gsm->rx_len;
    if (c >= 0x00 && c <= 0xFF) {
        // Process the received character
        gsm->rx_buf[len] = c;
        gsm->rx_len++;
    } else {
        // Handle invalid character
    }
}
```

**3. Checking for buffer overflow**
```c
static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
{
    unsigned int len;
    len = gsm->rx_len;
    if (len < GSM_MAX_RX_LEN) {
        // Process the received character
        gsm->rx_buf[len] = c;
        gsm->rx_len++;
    } else {
        // Handle buffer overflow
    }
}
```

**4. Validating the GSM structure**
```c
static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
{
    unsigned int len;
    if (gsm != NULL && gsm->rx_buf != NULL) {
        len = gsm->rx_len;
        // Process the received character
        gsm->rx_buf[len] = c;
        gsm->rx_len++;
    } else {
        // Handle invalid GSM structure
    }
}
```

**5. Checking for valid GSM state**
```c
static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
{
    unsigned int len;
    if (gsm->state == GSM_STATE_RX) {
        len = gsm->rx_len;
        // Process the received character
        gsm->rx_buf[len] = c;
        gsm->rx_len++;
    } else {
        // Handle invalid GSM state
    }
}
```

Note that these are just examples and the actual implementation will depend on the specific requirements of your GSM protocol and the structure of your `gsm_mux` data.