For gold, I use GLDM (SPDR Gold MiniShares) as the representative choice in the test portfolios, and hold it in real life, as well. Low cost (expense ratio: .18%) and simple.
Fund Home Page:

Barchart Page:

The Basics:
The thing about an ETF that tracks the price of physical gold bullion is that there really isn’t that much for them to do. Buy gold, store it, wait. Probably the only thing to think about is expense ratios, and GLDM has one of the lowest at .18%. A new fund, IAUM, was launched in June of 2021 with a slightly lower expense ratio (by 3 basis points), so that might be something to watch in the future, but for now, I’ve stuck with GLDM.
Investing in gold wasn’t always so cheap or easy. For most of my investing lifespan, I have been in GLD, which is the original State Street Gold Fund. That one does the same thing as GLDM, but for more than double the expense ratio. It seems a bit curious that the same fund company would put out two versions of the same thing, but from a little research, it appears they have different intended customers: “Due to its robust liquidity and tight bid/ask spreads, GLD is a favorite commodities ETF among professional traders and institutional investors, likely explaining why SSGA (State Street Global Advisors) and the WGC (World Gold Council) opted to introduce GLDM rather than paring GLD's expense ratio.” GLDM, then, is geared toward individual, “buy and hold” investors. Makes sense.
One thing to be aware of with this fund is that it represents actual, physical gold somewhere in a vault. At the launch of the fund, one share represented 1/100th of an ounce (that’s why its called a “mini” fund, since the regular ones hold 1/10th of an ounce). It is also possible to invest in gold by investing in futures, in the same way as the preferred commodities fund (PDBC) does. A fund following that path is Invesco’s DB Gold Fund (DGL). I don’t see much benefit either way, save for the expense ratios which tend to be lower with physical gold (DGL’s is .77%).
Correlation with Other Assets: Portfolio Visualizer Link
Gold is the quintessential non-correlated asset. No matter how many times I run a correlation matrix, it seems like I have never been able to see a number higher than .3 or lower than -.3 with any asset. You can see here the degree to which it is on its own path: .06 with the total stock market, .24 with Long-term Treasuries, and just .2 with other commodities. This data goes back to 2006, and as I recall, these results would not be all that different if you went out 10 or 20 more years.
Other Options:
As mentioned above, gold funds are all about the cost. For a long time, GLDM was the choice because it was the lowest in the space, but now there are some competitors: SGOL is at .17% and IAUM is at .15%. These could well be worth investing in, but as for my portfolio, I’d need to do a bit more research on them first.
Another option to invest in gold is via mutual fund, and I do this in my personal account with FKRCX (Franklin Gold and Precious Metals Fund), but I only do that because I can’t use fractional shares in one of my accounts, so it happens to be where I put odd, small amounts. It’s expense ratio is .9% (!) which actually makes it one of the cheapest precious metals mutual funds out there. The lesson there is to stick with ETFs, which are much more cost efficient. If you wanted to own both gold and silver in one fund, then perhaps Sprott Physical Gold and Silver Trust (CEF) might be a good choice, though its expense ratio is .5%. You could just get GLDM and SIVR instead, though.
Here is the link for a search of gold funds using the ETF Finder on Barchart. You might notice that GLDM is not on the list, since it is listed as a “Portfolio: Multi Asset” fund. This might be a classification mistake, unless there is something I’m not seeing. Anyway, here are the other funds:
