The trick here is to use a string format for the variable which forces the decimal point. Like so:

public float Length;

void OnGUI()
{
    string lengthText = GUILayout.TextField(Length.ToString("0.00"));

    float newLength;
    if (float.TryParse(lengthText, out newLength))
    {
        Length = newLength;
    }
}

EDIT: It is also possible by adding a bool to store if a . is entered. Downside is that it needs a further variable:

public float Length;
bool _isLengthDecimal;

void OnGUI()
{
    string lengthText = GUILayout.TextField(Length.ToString() + (_isLengthDecimal ? "." : ""));
    _isLengthDecimal = lengthText.EndsWith(".");

    float newLength;
    if (float.TryParse(lengthText, out newLength))
    {
        Length = newLength;
    }
}

when you trying to parse to float and parse value is not valid ("3.5g", "4.k3", ...), it throw exception you can write follow

try {
     someFloatMemberVariable = float.Parse(GUI.TextField(rect, someFloatMemberVariable.ToString()));
} catch {}