In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The in vitro data suggest that the site responsible for the decrease in seizure activity 24 h after gamma - HCH may be the GABA - A receptor - linked chloride channel .
### Output Text:
The in vitro data suggest that the site responsible for the decrease in seizure activity 24 h after <span class="chemical">gamma - HCH</span> may be the <span class="chemical">GABA</span> - A receptor - linked chloride channel . 