GPay is not a viable alternative for those based outside India. You cannot install the Indian version of the GPay app through the Android store (Google Play) without an Indian Google profile in place (which involves providing an Indian address, etc.). The standard US version of GPay / Google Pay / Google Wallet OTOH does not support UPI. Trying to get both set up to work simultaneously on an Android phone is possible but tricky and probably not an approach recommended for long-term use. No idea if this is even possible on a non-Android phone.

I do not have any recent experience with PayTM, though a few years ago it simply refused any non-Indian phone numbers so was a no-go.

Other than PhonePe, the best bet for those with US phone numbers linked to their Indian bank accounts to use UPI are the apps of the banks themselves. People have managed to get UPI working well with IDFC First, and HDFC. ICICI too but apparently it's more fiddly. SBI's app does not support full UPI for US phone numbers, but will allow payments to UPI addresses from your accounts.