Prevent using market-orders on gateio

GateIo does not support market orders on spot markets

tests/exchange/test_gateio.py

@@ -0,0 +1,28 @@
+import pytest
+
+from freqtrade.exceptions import OperationalException
+from freqtrade.exchange import Gateio
+from freqtrade.resolvers.exchange_resolver import ExchangeResolver
+
+
+def test_validate_order_types_gateio(default_conf, mocker):
+    default_conf['exchange']['name'] = 'gateio'
+    mocker.patch('freqtrade.exchange.Exchange._init_ccxt')
+    mocker.patch('freqtrade.exchange.Exchange._load_markets', return_value={})
+    mocker.patch('freqtrade.exchange.Exchange.validate_pairs')
+    mocker.patch('freqtrade.exchange.Exchange.validate_timeframes')
+    mocker.patch('freqtrade.exchange.Exchange.validate_stakecurrency')
+    mocker.patch('freqtrade.exchange.Exchange.name', 'Bittrex')
+    exch = ExchangeResolver.load_exchange('gateio', default_conf, True)
+    assert isinstance(exch, Gateio)
+
+    default_conf['order_types'] = {
+        'buy': 'market',
+        'sell': 'limit',
+        'stoploss': 'market',
+        'stoploss_on_exchange': False
+    }
+
+    with pytest.raises(OperationalException,
+                       match=r'Exchange .* does not support market orders.'):
+        ExchangeResolver.load_exchange('gateio', default_conf, True)