Do relational operators like less than, greater than, gte, lte use type conversion? If they do, why are there no strict comparison relational operators like !== or === ?
No, they aren't strict.
They can compare most types against each other, and cast as needed. For instance there's support for string comparisons:
$true = "a" < "b";
Internally that's zendi_smart_strcmp().
If you want to do strict comparisons, you can just check the types yourself, or cast as needed.