How to convert this cURL to wp_remote_get?

The curl is sending a POST request, to convert this you’ll want to use wp_remote_post(). As described in the documentation, the request arguments are the same as WP_Http::request(), so let’s look there how we can replicate your current version.

curl_setopt( $ch, CURLOPT_URL, $url ); just sets the URL. Already doing this with wp_remote_post()
curl_setopt( $ch, CURLOPT_RETURNTRANSFER, 1 ); tells that we want the result. wp_remote_post() is returning that by default.
curl_setopt( $ch, CURLOPT_POST, 1 ); makes sure to send a POST request. wp_remote_post() does that by default.
curl_setopt( $ch, CURLOPT_SSL_VERIFYPEER, false ); now this is an interesting one. Tells curl not to verify the https certificate. The corresponding argument that we need is sslverify.
curl_setopt( $ch, CURLOPT_POSTFIELDS, $curl_post ); specifies the fields to be posted. The corresponding argument that we need is body.

With this in mind, the result could look something like

$url="https://accounts.google.com/o/oauth2/token";
$curl_post="client_id=" . $client_id . '&redirect_uri=' . $redirect_uri . '&client_secret=" . $client_secret . "&code=" . $code . "&grant_type=authorization_code';

$args = [
    'sslverify' => false,
    'body' => $curl_post,
];

$result = wp_remote_post($url, $args);

This should give you enough to play around with. It might be more readable to send the data as an array:

$curl_post = [
    'client_id' => $client_id,
    'redirect_uri' => $redirect_uri,
    // ..
];

But that is up to you.

Check what exactly $result is. You might need to call json_decode() on its body.

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