In his time in charge of Wales, Gatland has led the side to Grand Slam success in 2008 and 2012 and most recently, took charge of the successful British & Irish Lions tour to Australia last summer.
On Friday night, he will coach in his 100th Test match as Wales look to back up last weekend's comfortable win over Argentina with victory against the South Sea Islanders.
"Warren is just a fantastic coach to work with," said former Wales scrum half Howley.
"Having been the other side of the line as a player, you can only look at the accolades he's won over the last 11 or 12 years - he's been a pretty successful coach. One thing you learn from Warren Gatland is he challenges you as both a player and a coach, he demands excellence and he knows what a winning environment is.
"He's a great person to work with and having spent the summer with him, there was no one more pleased in terms of the coaches he worked with on the British & Irish Lions tour to Australia.
"Warren is one to look forward not back and he's always willing to make hard decisions. That speaks volumes for him as a player and coach."
Unassuming Gatland has been a key figure in Wales' success over the last decade. On reaching the 100-Test landmark, the Wales head coach said: "It's a fantastic milestone but not something I had thought about until someone mentioned it to me recently."
While Gatland is playing down his own personal achievement, he will certainly be keen to see his side secure back-to-back Millennium Stadium wins. There are 11 changes in total from last week's victory over Argentina.
Howley said: "When you look back over the last three or four years, we've made many changes in terms of this fixture. It's always great when a new cap comes in.
"It's new territory for them tomorrow night and they're looking forward to it. The key thing for us is making sure there's experience in the spine of the team. There are some players who have been around our squad a while and we feel the experience in the pack will give us a good foundation.
"We started well against Argentina and we need a huge start against Tonga. They run hard and they'll prove difficult like any international side if we give them room. They have great footwork and ability to beat players one on one. Scrum time and line outs are going to be key areas of the game and the forward pack we've picked is pretty physical.
"It's about implementing our game on Tonga and I'm sure they'll have the same intention as us."
One man who will be key to carrying out Wales' game plan is James Hook, who starts at fly half for the first time since the 2011 Six Nations win over Scotland.
"The key for James is his decision making and the balance of his game," Howley added.
"James has been playing most of his rugby for Perpignan at full back - I think he's only started at fly half once and as a decision maker, you want to be playing 10 week in week out
"But he has to bring those mercurial skills which he has in the right areas of the game."