One day after I used a bacterial enzyme degreaser/ septic treatment to clear slow draining bathtub drains I can hear gurgling in my master bath tub drain when someone flushes the toilet in the other second floor bathroom. This morning the downstairs toilet was blowing bubbles when flushed. I have used this product before without this happening.
The gurgling typically happens with a vent pipe blockage. Without a source of air to follow behind the rushing water, a vacuum is created that will result in gurgling. You may be seeing that now if the vent pipe is partially blocked and there isn't enough air to support the faster flow. It may be necessary to snake out the vent pipe from the roof and/or run some water to clear out any debris that has collected inside.
Bubbles in a toilet below the water source (upstairs toilet) may be a sign of a blockage further down the drain. If that's the case, you may need your main drain line snaked and the plumber may need to run a scope down the line to find the location and cause of the blockage. It's possible that the enzyme pushed a clog further down the line, or the slow drains may indicate your septic system has started to backup into the house and needs to be serviced.
BMitch's answer is more likely to be your problem but ...
One other possibility: the trap is dry due to lack of use. It doesn't sound like your situation but if you have a tub, as an example, that isn't used often, the trap can dry out. Once it dries out, you'll hear a lot more noise from the plumbing. Simply put a quart of water down any infrequently used drains once a month to prevent this noise and to prevent sewer gas from entering through the dry trap.