You can call the action jumpDefinition with a command as argument:
nmap <silent> gs :call CocAction('jumpDefinition', 'split')<CR>
nmap <silent> gd :call CocAction('jumpDefinition', 'vsplit')<CR>
nmap <silent> gt :call CocAction('jumpDefinition', 'tabe')<CR>
To get more information:
:h coc-action-jumpDefinition