Why GMT and UTC timezones don't have same rules

The answer is in the JavaDoc of TimeZone#getTimeZone:

the ID for a TimeZone, either an abbreviation such as "PST", a full name such as "America/Los_Angeles", or a custom ID such as "GMT-8:00"

Returns: the specified TimeZone, or the GMT zone if the given ID cannot be understood.

And (from the class documentation)

[...] The syntax of a custom time zone ID is:

  CustomID:
     GMT Sign Hours : Minutes
     GMT Sign Hours Minutes
     GMT Sign Hours

The ID "UTC+5:30" is not a valid TimeZone ID (as per the specification of the method/class) and is interpreted as "GMT" zone, which is clearly distinct from the "GMT+5:30" zone.

Since you are located in India, you should use

    ZoneId india = ZoneId.of("Asia/Kolkata");

Two messages:

1. The TimeZone class has design problems and is long outdated. The same goes for its friends like Calender and SimpleDateFormat. So don’t use them. Instead use java.time, the modern Java date and time API. Its replacement for TimeZone is the ZoneId class (and ZoneOffset for an offset from UTC, but don’t use that as a time zone, it isn’t one, see the next item).
2. Don’t use an offset from either UTC or GMT as a time zone. It works in your particular case, of course, but it may leave the reader wondering why you chose +05:30, which Asia/Kolkata clearly conveys. Also Asia/Kolkata is future-proof in case at some point in time the politicians change the UTC offset for India or introduce summer time (DST). While this is unlikely for India, it happens all the time in other places in the world, so It’s better to make it a habit to use the region/city format always.

For just one of the many design advantages of the modern API, try the modern version of your code:

    ZoneId.of("UTC+5:30").getRules().equals(ZoneId.of("GMT+5:30").getRules())

This throws: java.time.DateTimeException: Invalid ID for offset-based ZoneId: UTC+5:30. Now you know from the outset what’s wrong: UTC+5:30 is not a valid time zone ID.

Link: Oracle tutorial: Date Time explaining how to use java.time.