Hi,
currently it is like this:
  static GEnumValue modulation_types[] = {
    {QPSK, "QPSK", "QPSK"},
    {QAM_16, "QAM 16", "QAM 16"},
    {QAM_32, "QAM 32", "QAM 32"},
    {QAM_64, "QAM 64", "QAM 64"},
    {QAM_128, "QAM 128", "QAM 128"},
    {QAM_256, "QAM 256", "QAM 256"},
    {QAM_AUTO, "AUTO", "AUTO"},
    {VSB_8, "8VSB", "8VSB"},
    {VSB_16, "16VSB", "16VSB"},
    {0, NULL, NULL},
  };

It should be more like:
  static GEnumValue modulation_types[] = {
    {QPSK, "QPSK", "QPSK"},
    {QAM_16, "QAM 16", "QAM-16"},
    {QAM_32, "QAM 32", "QAM-32"},
    {QAM_64, "QAM 64", "QAM-64"},
    {QAM_128, "QAM 128", "QAM-128"},
    {QAM_256, "QAM 256", "QAM-256"},
    {QAM_AUTO, "AUTO", "AUTO"},
    {VSB_8, "8VSB", "8-VSB"},
    {VSB_16, "16VSB", "16-VSB"},
    {0, NULL, NULL},
  };

Enum nicks usually contain no spaces and are usually all lowercase as well. Every other element I'm aware of uses the same rules as the rules that apply for property nicks.

Problem with spaces is especially when using gst-launch, you need to use modulation="QAM 16" instead of something like modulation=qam-16  (note the "")

Would be nice to change this here before it gets moved to -good :)