Here is the error message appearing when trying to upgrade the qtype_gapfill plugin to 1.4 using Moodle admin account:

Debug info: SQLState: 42000<br>
Error Code: 156<br>
Message: [Microsoft][SQL Server Native Client 11.0][SQL Server]Syntaxe incorrecte vers le mot clé 'column'.<br>

ALTER TABLE mdl_question_gapfill add column disableregex tinyint(1)
default 0 after noduplicates

Error code: ddlexecuteerror
Stack trace:
line 432 of \lib\dml\moodle_database.php: ddl_change_structure_exception thrown
line 260 of \lib\dml\sqlsrv_native_moodle_database.php: call to moodle_database->query_end()
line 707 of \lib\dml\sqlsrv_native_moodle_database.php: call to sqlsrv_native_moodle_database->query_end()
line 79 of \question\type\gapfill\db\upgrade.php: call to sqlsrv_native_moodle_database->change_database_structure()
line 473 of \lib\upgradelib.php: call to xmldb_qtype_gapfill_upgrade()
line 1544 of \lib\upgradelib.php: call to upgrade_plugins()
line 349 of \admin\index.php: call to upgrade_noncore()

This SQL query is not compliant with SQL Server. I've spotted two reasons why:

1) On SQL Server, an ADD COLUMN statement must be wrote without the 'column' key word like this:
ALTER TABLE mdl_question_gapfill ADD disableregex tinyint(1)
default 0

2) SQL Server isn't compliant with 'AFTER' keyword.