GIFs have an Image Mode of
Indexed Color (max 256 colours). My guess is that your Photoshop document is
RGB (usually the default for new PSDs). You need to change your GIF to RGB (to match the Image Mode of your target PSD) as follows:
- Ensure your GIF is the active image
- Select Image > Mode
- Indexed Color will currently be checked. Click RGB Color to change Image Mode.
Job done.